In The Prince, Machiavelli says that in general, humans are

ungrateful, fickle, false, cowardly, covetous . . .

Therefore, a wise prince must be prepared for the fickle or changeable aspect of human nature. Machiavelli goes on to say that because humans are fickle or untrustworthy

the prince must consider, as has been in part said before, how to avoid those things which will make him hated or contemptible . . .

In Julius Caesar, we see how the very Machiavellian Mark Antony makes mincemeat of Brutus, showing the truth of Machiavelli's observation that people (the "mob") are fickle.

In his speech to the mob following Caesar's death, Mark Antony easily sways the crowd, who, moments before during Brutus's speech, were ready to crown Brutus emperor (even though, ironically, Brutus claimed he killed Caesar to save Rome from that fate).

Mark Antony manages to sway the crowd against Brutus by showing contempt for him. Through repetition, he makes ridiculous Brutus's claim that he is an "honorable" man. By the time Antony is done with the crowd, they are raised to a pitch of anger at Caesar's murder and ready to turn on Brutus and his followers. Civil war ensues.

Julius Caesar illustrates Machiavelli's point that a successful prince must at all costs avoid whatever will make him appear hateful or contemptible. Brutus's sense of honor is his undoing: Antony's ruthless pursuit of power and spot-on manipulation of the crowd, in contrast, serve him well.