What makes Antony in Julius Caesar a villain?
Let us remember that Antony is supposedly a staunch supporter of Caesar. However, what he does after Caesar's death is to deliberately whip up the crowd of Roman citizens with his speech so as to unleash a riot on Rome and defy the conspirators. He shows himself to be just as manipulative and power-hungry as the conspirators themselves. Note what he says when the Citizens leave the scene determined to start a riot:
Now let it work: Mischief, thou art afoot,
Take thou what course thou wilt.
He deliberately sets fire to the tensions in the Roman populace and then sits back to watch what will happen.
Perhaps most importantly in this play however is Act IV scene 1, which clearly shows Antony's bloodthirsty and power-hungry nature as he calmly discusses with Octavius and Lepidus who needs to be killed as part of their rise to power. What is most chilling is the way that he consigns his nephew to death without even a qualm of conscience:
He shall not live; look, with a spot I damn him.
Antony is clearly a man that is determined to gain power and will not let anything stand in his way. Another moment that shows his villainy is when in the same scene he immediately tries to undercut the authority of Lepidus with Octavius, arguing that they should dispose of him and share the power between them. This of course shows him to be a treacherous dishonest individual. In short, such evidence points towards the way in which Antony is even worse than the conspirators himself, and certainly does not have the high-minded ideals and justification of Brutus.