Basil - during his lifetime - was undoubtedly one of the most successful emperors of the Middle Byzantine period. He recovered/ reconquered a huge amount of external territory (Bulgaria, of course, but also in Armenia, Iberia, and parts of Sicily and Southern Italy) and took steps at home to bring unruly elites to heel. At his death in 1025, the empire was relatively strong: a competent military, a relatively abundant treasury (especially impressive since he fought so many wars), a complex and vibrant culture. After he dies, though, his successes don't last, and issues he left untreated or patch-fixed develop into more serious problems. Is Basil directly responsible for, say, Manzikert? I mean, probably not. But is he responsible for the extinction of the Macedonian dynasty, which led to weakened emperors, diminished infrastructure in the eastern empire, and uncoordinated defense against the Turks? Well, that connection is a bit easier to make.

Basil was militarily successful during his lifetime (at least partially) but his conquests/reconquests, successful or otherwise, cause Byzantium a great deal of stress after his death. The empire was already overextended when he rose to the throne, but his annexation of Bulgaria only exacerbated the shortages of material and manpower (to say nothing of talent) that had earlier in the 10th century let usurpers like Romanos I or "regents" like John II Tzimiskes accumulate so much power. Following a terrible defeat at the hands of Samuel of the Kometopouloi (986), Basil spent so much time and effort on Bulgaria that he essentially ignored the empire's eastern frontier. To be fair, at this point there was no serious threat to Constantinople from Anatolia - unlike the Bulgarians, Pechenegs, etc. in the European half of the empire - but after his death the weakened frontier defenses and damaged cohesion of the great noble families in Asia minor, who might have been able to form a solid, united front, let the Turks roll right in. Basil had also managed to antagonize most of the elites in Anatolia (dynatoi) for various reasons. This dislike for powerful nobles may have stemmed from the revolts of Bardas Phokas and Bardas Skleros Basil had seen in his youth; nevertheless he made clear attempts to remove the old elite families (e.g. Skleros) from power and install new ones loyal to him (e.g. Dalassenos, Komnenos - which then, interestingly, go on to produce Alexios I) in their stead. Basil passed several reforms limiting the ability of these elites to intervene the affairs of peasant landholders. Starting in the early 10th century we start to see powerful families developing the practice of buying land from peasants in exchange for assuming the duty of paying the tax on that land (substantial to the peasants, but almost meaningless to a sufficiently wealthy family). This granted the new landowners incredible resources which they might use to rebel against the emperor - definitely something Basil wanted to avoid. He also forbade these magnates to purchase peasants' lands used to support soldiers (stratiotika ktemata) which would have allowed them to weaken the imperial military while simultaneously building up their own armed forces - something, again, they might have used to rebel. He supplemented the slightly diminished military support of these disgruntled nobles with external forces like the Venetian navy and the Varangian guard, which naturally only alienates the native Byzantine elites even more.

One of Basil's biggest flaws, however, is that he left no heirs. He spent so much time shoring up his position (admittedly after having to seize his own power back from a series of regent-usurpers) and on military campaigns that he completely neglected domestic affairs; he never married. This meant the throne went to his brother, Constantine VIII. Constantine had only daughters -- Eudokia (a nun), Zoe, and Theodora -- but he was only a few years younger than Basil, who died at 67. There was no question of Constantine producing a male heir before his own death in 1028. Constantine, moreover, was an incredibly weak emperor. Though theoretically Basil's co-emperor (since 962), he lived a life of luxury in Constantinople while Basil was out on campaign. Once he became sole emperor he was not a very good ruler: he punished crimes almost indiscriminately with blinding, set to work on Basil's savings through lavish living and unrestrained generosity, and was (predictably) allegedly controlled by his eunuchs. On his deathbed, he scrambled to find someone suitable to marry his daughter Zoe and thus to succeed him as emperor. He finds someone - Romanos III Argyros, formerly the Eparch of the City - but the fact that neither Basil nor Constantine could ensure a direct, blood succession is the death knell of the Macedonian dynasty. With marriage to Zoe - who was at this point past childbearing age anyway - as the only possibility for further Macedonian emperors, this lack of a male heir kicks off the succession issues of the early eleventh century.

Basil was able to project (and probably actually accrue) a great deal of authority partly because he reigned for an incredibly long period of time. Even as sole emperor (i.e., without regents like John Tzimiskes, Nikephoros Phokas, or Basil Lekapenos) he reigned for almost fifty years (976-1025) - longer than Augustus, whose name the Byzantine emperors used as a title. Constantine, his successor, only lasted three years; Romanos lasted a little longer (six years, 1028-1034), and his successor, Michael IV, lasted just as long (1034-41); but Michael's successor (and nephew), Michael V, lasted five months (December 1041-April 1042) before being violently deposed, blinded, and exiled. After that, Zoe and Theodora rule jointly for four months, until Zoe takes another husband, Constantine IX Monomachos, who holds on for a little over ten years (1042-1055). Zoe had died in 1050, so after Constantine IX the throne goes to Theodora, who only lives for another year. She names a successor, but the Macedonian dynasty ends with her. Between her death and the accession of Alexios I, no emperor lasts even as long as Constantine IX. It's significant, I think, that that Alexios I (1081-1118) is the first emperor since Basil II to reign for more than thirteen years, and the next two emperors succeeding him rule for comparably long periods (John II: 1118-1143; Manuel I: 1143-1180). Would all of this had happened had Basil II had a son? Of course that's impossible to say (or at least a question for another subreddit). But the absence of a male Macedonian heir led to almost a century where only one emperor ruled for more than ten years. This made the empire vulnerable to attack from both exterior and interior forces: the former exemplified by the battle of Manzikert in 1071, and the latter by Alexios I's usurpation ten years later.

Recommended further reading (if you're still interested after this wall of text!):

• Michael Psellos, trans. E. R. A. Sewter, Chronographia [Fourteen Byzantine Rulers] (Baltimore: Penguin, 1979). An earlier edition is also available online. An extremely entertaining read; I 100% recommend it. Psellos relates many of the problems following Basil's death, at least those that affected his cushy life at court. Basil's reign and character are covered in Book I.
• Oxford Dictionary of Byzantium, s. v. "Macedonian Dynasty." Mostly for the very helpful diagram of the dynasty's progression, especially as it disintegrates after Basil's death.
• Stephen Kamer, "Emperors and Aristocrats in Byzantium, 976-1081" (Ph.D. Diss.: Harvard, 1987). Examines the dynamic between Basil and the elites he so frequently fought with, which (as you can probably tell from this answer) I'm not 100% familiar with.
• Catherine Holmes, Basil II and the governance of Empire (976-1025) (New York: Oxford University Press, 2005). Holmes helpfully sifts through the rather muddled primary sources on Basil's reign (even Psellos admits he was only a child when Basil died), and pays careful attention to his interaction with elite families in Anatolia, though not in the context Kamer does.

How dare you slander my name like that!

Now for actual work

Throughout the study of Roman and Byzantine history, historians, due to their own nature as sort of the great “reality check” of the past, have gone back and fundamentally re-evaluated the rules of many “Great” Emperors. Where once Manuel I was Ho Megas (The Great), current scholarship now places much of the blame of the eventual collapse of the government to the Turks on what used to make him be considered great, his escapades and grand campaigns against the Hungarians, Egpytians, Normans, etc. Likewise, Michael VIII, the great restorer, was once viewed as a tragic hero who was a good emperor with a crumbling Empire, while now modern scholarship views him as a well-intentioned Emperor whose Empire was actually in pretty decent shape, while Michael VIII’s neglect of the Anatolian frontier in favor of the European front eventually led to the collapse of Anatolia. This process is powerful and has fundamentally changed our view of these great Emperors. The power and inevitably of this process, to me, is what makes Basil II so unbelievably interesting and powerful: despite 1000 years of scholarship, there simply isn’t much legitimate or well-regarded scholarship to say Basil really did all that much that was bad. So, to answer your question shortly: No, it’s not really fair to put blame on Basil II as the “architect” of the decline of the Byzantine Empire.

For the simple “these are things he did” type facts, I’m referring, unless noted, to the only real “complete” narrative of his reign, John Skylitzes “Synopsis of Byzantine History,” although I’ll note the issues of the text later. I’ll mark other references as necessary.

To begin with, I’ll go through some of the really un-arguably good things he did for the Empire. Let’s first take a look at the treasury. To the Byzantines, who relied on a huge market economy and coinage, the fullness of the Imperial treasury was often a way to measure how effective an Emperor was, since that treasury was the source of the money that the Byzantines paid their all-important army, navy, and, most importantly, where they got the money to pay off their enemies as tribute (The importance of Byzantine tribute I go more in depth to here). Basil II was able to, by the time of his death, increase the imperial revenue to about 7 million nomismata (or about 100,000 pounds of gold) and left the treasury full to the brim with 14.4 nomismata, so about 200,000 pounds. This was truly an enormous sum, Justinian’s “great bribe” to the powerful Persian Empire, a polity also capable of raising enormous sums of money, was 11,000 pounds of gold. Basil II didn’t even do this in overly-oppressive ways, Skylitzes reports that Basil allowed many of his subjects to pay taxes in kind (I’ll speak more of this later), and Basil’s reign also saw a “golden age” of sorts for lower-class farmers and peasants (This is potentially, actually, one source of trouble after Basil II’s death that I’ll report on later). Basil, despite the fact that he had to spend enormous sums of money to finance his vigorous and never-ending wars, was effectively able to leave his successor a full two years worth of revenue in reserve, something that is almost unheard of in Byzantine and Roman history.

Now, let’s look at the borders. The history of Roman borders is one of natural defenses, the Romans were always interested in expanding to the point where their Empire was easily defendable due to natural barriers, and Basil was no different. In the west, he pushed the Empire to the Danube, a powerful river that would effectively act as a barrier for the vast majority of the next 100 years until the revolt of the Assen brothers under Isaac II.^1 In the East, Basil pushed the borders of the Empire to the incredibly mountainous regions of Armenia and lake Van which, together with the extensive fortifications Basil commissioned, formed an incredibly effective barrier against southward movement by steppe peoples were it not for the overall incompetence of the later Macedonians. It was a long frontier, and the Empire could never hope to man ALL of it, but the blessing of mountain ranges is that they form small gaps that are easily defensible by a small number of troops. Against the Arabs Basil pushed the borders to the Euphrates, and where he didn’t have natural borders he envassalated a multitude of states such as Aleppo and Edessa, making political buffers where ones of water and sand did not exist. These weren’t Justinian type expansions where they were almost impossible to hold either, Catherine Holmes in her Basil II and the governance of Empire, for example, argues that there’s very little reason to believe that these borders shouldnt have given the Byzantines supremely defensible and easy to manage territory. ^2

Finally among the almost completely positive things about his reign was Basil II’s diplomacy. His reign saw the apogee of Byzantine political and cultural influence in Europe, and as such this arguably his most successful sphere as a ruler. To begin with, his sending of Anna, his sister, to marry Vladimir the Great meant that Rus, a rising power in the east, would remain allies of the Byzantines and not the West, of particular note due to its allowing of the foundation of the Varangian guard, an elite unit of Scandinavian Warriors that would provide some of the Empire’s best troops. He brought Venice firmly into the Byzantine cultural sphere as well. Whether or not this was a deliberate action or one of reaction is up to debate: Skylitzes paints it as Basil simply being a genius while more modern historians, such as Jonathan Shepherd, paint it as more of Basil II being opportunistic and fortunate moreso. Honestly, it’s most likely the latter, Basil II most certainty wasn’t thinking of the long term effects of sending his sister to the Rus, but that lack of long-term planning certainty shouldn’t take away credit from him.^3

So, on a superficial level, he only enriched and made more powerful the Empire, so now I’ll delve into some of the things that arguably at least made Basil the progenitor of Byzantium’s later problems.