Who says he wasn't hamstrung by treacherous people in his own Witan/Council? By the 11th century, England was Anglo-Norse in a way, since by then many Norse had settled and had become English in mannerisms and culture. Even during the Great Heathen Army's time, many years before Athelred lived, his great-great-grandfather King Alfred the Great had to contend with English collaborators. Edmund and Edred, his grandfather and great uncle and predecessors as King of the ENglish had to also.
So as collaboration was entrenched, and by that time even longstanding English lines had Norse branches or relatives,the scope for treachery had large scope.
Maybe he was ill-advised or "unready" because he knew he could only act as far as he could?
An example is Eric Bloodaxe, who had native English support in taking York. Or Tostig Godwinson, whom allied with King Harald Hardrada,or Edric Streona who colluded with Cnut against Athelred 's son King Edmund Ironside, after he died at Assandun.
I'll paraphrase an answer I gave to an earlier similar question.
His name and nickname, rather hilariously, mean Nobly Advised, The Ill-Advised. We know from the ASC that Æthelred had massive problems keeping his nobility in line, since many of them had preferred his brother and saw him and his mother as usurpers. This level of discontent was far higher than, say, under Edgar or Eadred. Many of the nobles that should have been co-operating for the defence of England fell into squabbles and infighting as well, meaning that instead of uniting their forces to fight Cnut's armies, they fought each other and let the Danes and Norwegians just swan straight in. Æthelred is seen as weak precisely because he wasn't able to unite the nobles in the same way that Alfred, Edward or Æthelstan were. The English do score some victories against the Vikings in this period, but these tend to come from successful defenses by the fyrd and local ealdormen. In comparisson, royal campaigns tend to fizzle out, like Æthelred's abortive campaign against Man, when half the fleet he's expecting just doesn't turn up.
The writer of the Anglo-Saxon Chronicle is very bitter about all this. In 1006 he cracks a very black joke about how the Vikings 'are left to enjoy their usual amusements', and do the English the courtesy of 'lighting' the early-warning beacons by burning down villages, since the organised defensive system that was meant to be in place has been allowed to collapse into anarchy. So this could mean he was unræd in being poorly supported as well as advised.
He also made some very contentious and unpopular decisions. In particular there's one ealdorman that just keeps betraying him but he keeps trusting. At one point, he has the guy's son blinded as a punishment for his betrayal, and then less than a year later is surprised when the guy suddenly doesn't sent troops to help the king's army. Alfred, on the other hand, is known to have replaced ealdormen who weren't up to the job, at least according to Asser. Many nobles also disagreed with his policy of re-establishing the Danegeld to try and pay the Danes off. The Bishop of Worcester writes him a letter complaining that his estates are being bled dry to pay for the new tax. And then there was the St Brice's Day Massacre which is roundly a terrible idea.
S: Dorothy Whitelock's 1961 translation of the Anglo-Saxon Chronicle
Roach, L. [My supervisor!] (2016), Æthelred the Unready