I've seen Julius Caesar reffered to as a "villain." Why is that?

Julius Caesar was the key player in the end of the Roman Republic (closely followed by the birth of the Roman Empire after his death). The Republic had been in political turmoil for some time, beginning years before Caesar was even born, and he marked the end. He was a very capable general, his soldiers adored him and he pretty much did not see a defeat, even against larger and more powerful armies. He conquered all of Gaul for Rome. He was a very successful politician, he championed the people's rights against the elite nobles. He rose quickly in the political ranks. In the end, he took up arms against the government that he said was illegally trying to suppress/eliminate him, took control of Rome, defeated the other faction in civil war, and ended up declaring himself dictator for life, consolidating all the power in Rome. At the end, he was assassinated, his assassins said they were killing a tyrant and freeing Rome.

Throughout history, people have looked at these same events, and some have seen Caesar as a hero that fought for the people and Rome, and others as a villain that brought down the Republic. I will first give a short list of these contrasting viewpoints, then show how the main actions of Caesar can be seen in either light.

People's Conflicting Views of Caesar

[+] 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.
[+] 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, instead of thinking of Caesar within the polarity of achievement vs ambition, thought of him as an earthly counterpart to God.
[+] 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.

Good or Bad? Gallic Wars: Conquerer of Gaul

Caesar arranged himself to be proconsul of Transalpine Gaul for a while (partly because of the immunity the positon gave him -- he had political opponents that were waiting for him to step down from political office so they could prosecute him, partly because wars he was planning to wage here would be very profitable and he was in a lot of debt). As he describes in his The Gallic Wars, every year he had campaign after campaign against the Gauls, eventually conquering all of Gaul. He showed great skill as a general. Some people have described this as an amazing feat in bettering Rome, helping Rome's friends and basically expanding Rome in self defense against enemies. Others have described it as genocide against the Gallic people, and the reasons Caesars gives for each campaign as made up / exaggerated / manipulated. This same camp will also say that The Gallic Wars, Caesar's own description of these wars that make him shine as a hero, is written for propaganda purposes in a very narrow point of view to promote him among the people.

Good or Bad? Politics and Civil War: Champion of the People

The politics of the era was pretty complex, but let me briefly summarize main ideologies. People were either patricians (privileged aristocrats) or plebs (the rest of the people). Actually by first century these ancestry-based distinctions were blending away, but the idea of the elite (nobilis) and the mob/people (populus) was intact. There had been two main ideological factions in the political climate for a while:

Populares. Movement for The People. Populist politics. Main issues: (more rights to the people, lowering prices of bread through use of government funds, make all immigrants/allies citizens, veto powers to people's direct representatives for stopping aristocratic senators passing laws that benefit themselves, erase all debts, redistribution of land to the poor, less moral oppression in areas like marital affairs and sexual freedom...) Examples: Gracchi brothers, Marius, Cinna, Clodius, Caesar.
Optimates. Traditionalist senatorial majority. Main issues: (opposing erasing debt or constant monetary assist to people under the belief that despite the short term satisfaction economy and stability will be hurt way worse in the long term, opposing too much veto/lawmaking power to people's representatives as they only go for popular policies that gather support rather than an in-depth analysis of what's best for Rome, traditional and conservative values and virtues such as honor, loyalty, marital fidelity, justice, duty and respect to gods, etc.) Examples: Sulla, Cicero, Cato.

You can think of populares as the occupy wall street movement and optimates as the lobbying 1%, or you can think of populares as the Communist Party USA at the height of its power, and optimates as the American government trying to uphold American values against this wave. There are many more differences than similarities to any of these comparisons, but I provide such shallow comparisons to reinforce the idea that depending on where you are looking from, either ideology can be seen as heroic or villainous.

Caesar has championed or exploited (depending on your view) the populares faction, and rose quickly in the system as a smart and capable politician. He used brilliant power balance structures (such as the First Triumvirate), enourmous amounts of bribes in corrupt elections, timely alliances and political attacks to defeat his enemies, and when the time came, use of his armies and capabilities as a good general to take power by force. In the end, he was dictator for life, and was at the top of the power pyramid, consolidating all power in Rome.

If you favor the ideals he championed, you can see this as necessary for the righteous and true values to be brought to Rome, and see him as a hero. Another shallow comparison would be the French Revolution: When people revolted, they were acting against the laws of the current government, but they were doing so in the name of liberty, equality, and freedom. If you believe these ideals, you won't see the breaking of the law as negative. Similarly (yet again, of course in a very different way), even though Caesar led an armed opposition against the Republic and took control, you don't need to see that as villainous if you believe his ideals behind these actions and think it is not mostly personal ambition.

Conclusion

I heartily recommend you to read more sources to see the complexities for yourself and make up your own mind as to what degree Caesar was good or bad for Rome's present and future. I think you are more likely to see that a hero vs villain spectrum is an oversimplification. Many key characters in the story such as Caesar, Pompey, Crassus, Cicero, Cato the Younger, Marcus Antonius, Brutus, etc. were not fighters in a clear cut war of light vs dark | freedom vs tyranny | the elite 1% vs the people, but all complex characters with different ideals and motivations, whose paths often crossed each other, both in conflict and mutual benefit. The end result was the fall of Roman Republic by Caesar's hand in the end, and the birth of Roman Empire by the hand of his grand-nephew he adopted as a son, Gaius Octavius.

EDIT: Spelling.

EDIT 2: Removed suggestions of podcasts as asked by mods.

EDIT 3: Correction by /u/zamieo : Octavius was Caesar's grand-nephew, not cousin.