While the optimates such as Pompey or Cato are typically depicted as 'bad guys'? This seems to be the case in fiction such as the TV show Rome, or even the "Masters of Rome" series (excellent books BTW). However even in earlier times there seems to be this perception, with Dante relegating the killers of Caesar to the worst pits of hell.
It's actually a case of much older fiction than that. But we can start with Dante. Dante Alighieri's famous epic The Divine Comedy specifically glorifies Julius Caesar as one of greatest Roman citizens. Indeed, in Dante's depiction of the devil, Satan is eternally chewing on Judas, Brutus, and Cassius. Two of the three betrayed Caesar, while one of them betrayed Christ. That alone should indicate the relative significance of Caesar in the early Renaissance. Caesar himself is featured earlier in the work, in the first circle of Hell, called Limbo. Limbo, according to Dante, is reserved for virtuous non-Christians (as anyone not baptized in Christ is denied Paradise). Caesar is joined by other figures from antiquity who would be well respected by Italians of Dante's time, such as Homer, Socrates, Aristotle, and Cicero.
If you want to go further back, Virgil was doing his own work making Julius Caesar a national hero. Between fifteen and twenty-five years after Caesar's death, Virgil wrote his own epic, The Aeneid, modeling it in structure and style after Homer's two great epics. (Significantly, Dante would appoint Virgil as his fictional guide through Hell and Purgatory in the first two parts of The Divine Comedy.) In Virgil's epic, Ascanius, Aeneas's son, is renamed Iulus, which designates him as the ancestor of the house gens Julia. This means that all of the Julii are directly descended from the royal house of Troy, seen as a prestigious linage.
(Interestingly, Virgil isn't the only writer to want to connect his hometown to the glory of Troy. Geoffrey of Monmouth, in his Historia Regum Britanniae, wrote about Aeneas's great-grandson Brutus who, after some wandering, settles on the island of Albion and renames it after himself: Britain. To bring this full circle, Geoffrey's history later depicts Julius Caesar's invasions of Britain and his ultimate truce with Cassivellaunus, then King of Britain and probably descended from Brutus. So taking both Geoffrey and Virgil at face value, Cassivellaunus and Caesar were distant cousins, both descended from the kings of Troy. This is a good reason not to take largely fictional histories seriously.)
Actually, throughout history, there have been plenty of both heroification and villification of Caesar. In another answer to a similar question, I go over the details.
Basically, it is entirely possible to interpret Caesar's actions as either good or bad, depending on your political (and historical) position. Over time, there were many that put him up on a pedestal as a champion of the people and the instigator of a stable empire, many that hated him and cited illegal wars, tyranny, and the fall of Roman republic, and of course some that tried to remain neutral and stick to the facts. One viewpoint may dominate at a given place for some (relatively) short amount of time, but I don't believe either type of portrayal has disappeared to this day.
#Love and Hate towards Caesar throughout History
Here are examples of such opposite views from some famous figures.
[+] Ovid, 50 years after Caesar's assassination, wrote of him as the shift from chaos to order.
[-] Lucan, in AD 65, was much less favorable of him in his poem of the civil war, Pharsalia. Caesar is depicted as as an all-too-brilliant villain, an extremely capable enemy. You can sense respect in Lucan's (a republican) hatred.
[+] Suetonius saw him delightfully as a great achiever.
[-] Plutarch saw him as a man of destructive ambition.
[+] Augustine saw him as the embodiment of the divine destiny of the Roman Empire, simply inevitable.
[-] Thomas Aquinas in 13th century said that Caesar's murder was justified because he took power by violence.
[+] Dante, as you mention, instead of thinking of Caesar within the polarity of achievement vs ambition, thought of him as an earthly counterpart to God. An unstoppable force of destiny.
[+] Machiavelli praised his achievements and military prowess.
[+] Montaigne was impressed by the power of his charisma and presence, as well as his excellence as a general.
[+-] Shakespeare's take on him is of course analyzed in-depth in many other places.
[-] Francis Bacon felt that Caesar cared more about his personal advancement than the good of the people, and in the end his extensive popularity brought his own demise.
[+-] Milton was on the side of seeing him as a tyrant, but he also admired Caesar's achievements: "Indeed if any tyrant were to be spared, I would wish it to be he: for although he rushed a kingship upon the republic somewhat too violently, yet he was perhaps most worthy of kingship".
[-] Hobbes says that he won the affections of his army and the people, so the people set him up against the senate, and he made himself master, of both the people and the senate. This, he says, is plain rebellion, and as bad as "the effects of witchcraft".
[+] In 17th century France, comparisons between two mythical figures, Alexander the Great and Caesar became common. Many thought Alexander was a hero but Caesar was the greater man.
[+-] Montesquieu admired both, but preferred Alexander. His ideals were closer to Cicero, hence at odds with the final chapter of Caesar's politics.
[-] Rousseau was even more Ciceronian, and thought of Caesar as a terrible tyrant.
[+-] Voltaire and Frederick the Great respected Caesar's greatness, but still condemned some of his actions, especially the final dictatorship and consolidation of all power by violence.
[+] Napoleon admired Caesar and compared himself to him. He thought of Caesar as an example and forerunner.
[+] The German Romantic Scholar Gundolf, around 1925, saw him as a great, mythical man, a symbol of successful imperial expansionism, a Nietzschean Ubermensch.
If you'd like to check out some of these different depictions firsthand, here is where you can find them.
I would be curious to know if Caesar's reputation was due to propaganda by his heir, Augustus. It would have been in his interest for Caesar to have been a great man, Augustus had a long reign and a lot of time to drill that idea home, and, the men who murdered Caesar, ie, the opposing view, didn't last long.
As a Latin student, while talking about Caesar's Gallic Wars, we always talked about how Caesar used the work as propaganda to shift all blame away from him when something went wrong and to say he was 100% responsible for all Roman successes in Gaul. Would medieval and renaissance people thought of this as a character flaw in Caesar or is this a newer interpretation?
by Roman standards, Caesar was a virtuous person. Patrician, a great general, and a great leader who granted got too big for his boots and died as a result.
And to be fair, he did set into motion the start of the Roman Empire, even though he was never an Emperor himself.