The theme of "the ends justifying the means" is one of the most important in Julius Caesar. The entire play hinges on the concept that a single man's life might not be as valuable as the greater good of society.
Caesar, the Dictator of Rome, is not a bad or evil person, but he does have an inflated opinion of himself. He is also almost a minor character in his own play, only appearing in three scenes. While he is a powerful man in Rome, he is also prone to overconfidence in his own abilities; Cassius tells Brutus about Caesar's attempt to swim across the Tiber river, and how Cassius had to rescue him when he became unable to continue. This weakness, Cassius determines, is the forewarning of a vanity that will lead to corruption if he is allowed to continue gaining power.
In the meantime, Caesar grows more and more confident in his own abilities, even while he ignores the possibility of plots against him:
Caesar: Caesar shall go out. The things that threaten me
Never look on anything but my back; when they see
The face of Caesar, they disappear.
Danger knows full well
That Caesar is more dangerous than he.
We are two lions born on the same day,
And I am older and more terrible;
And Caesar shall go out.
After Caesar is murdered, Brutus sums up his reasoning thus:
Brutus: Not that I loved Caesar less, but that I loved Rome
more. Would you rather Caesar were living, and you all die slaves, than
that Caesar were dead, so you all live freemen?
His belief that his ends -- the freedom of Rome -- justifies the means -- the murder of Caesar -- is how he justifies his own actions, although after Antony speaks, the crowd rejects Brutus and seeks to revenge Caesar's death.