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.
Machiavelli's observation, especially in Chapter 18 of The Prince, is that successful leaders have been those able to change their attitudes with the wind, and moreover, to appear to be virtuous, moral, and religious without actually being so. It's clear in Julius Caesar that the conspirators, among them chiefly Brutus, are unable to act this way. Brutus is too honest a man to be an effective ruler, and besides, it's not his ambition to "rule" anyway. The Prince, as its title indicates, and as Machiavelli himself states, deals with how princes, those who rule by hereditary right and hold absolute power in their realms, have most effectively governed and have been successful throughout ancient and modern times. He deals with the quite different situation of republics elsewhere, in the Discorsi. Brutus was a man committed to restoring the Roman Republic which had essentially, though not officially, been brought to an end by Caesar's dictatorship. Brutus is clueless as to how to manipulate the crowd when he goes into the pulpit to address them.
Yet at first, his words seem to have an effect on them, as they cry, "Live, Brutus! We'll make him a king!" The enthusiasm of the crowd for him lasts about five minutes. Though at first they don't wish to hear Antony, as soon as he begins appealing to the emotions of the people and their visceral connection to Caesar, whom they see as having been a great benefactor to them, everything is reversed and there is no sympathy for Brutus and the conspirators. As has occurred throughout history, the emotion of the crowd turns on a dime (or on a denarius, we might say!) and now everyone is out to avenge Caesar's death.
This fickleness of "the people" is precisely what Machiavelli says has been exploited by successful princes, who themselves have cynically shifted their stances and alliances in order to hold onto power. Unfortunately for Brutus and the conspirators, there is no opportunity for them to do so. Events overtake them so quickly that as soon as Antony has completed his speech, he has "let slip the dogs of war," and there is no going back. Shakespeare's depiction of this is telescoped. In actual history, it took at least a week after the assassination for growing resentment and anger against the conspirators to reach the point where they basically had to get out of Rome as fast as they could. But the general point is clear about how easily "the mob" can be manipulated, and how emotion holds sway over reason--phenomena which we see in our own time as well.