In Act III, Scene II- Brutus tells the people that he loved Caesar, but he loved Rome more. He killed Caesar for the good of Rome. If Caesar had lived, he would have made the Romans slaves.
"If then that friend demand why Brutus rose against Caesar, this is my answer,--Not that I lov'd Caesar less, but that I lov'd Rome more. Had you rather Caesar were living, and die all slaves, than that Caesar were dead, to live all freemen? As Caesar loved me, I weep for him; as he was fortunate, I rejoice at it; as he was valiant, I honour him; but, as he was ambitious, I slew him."
Antony speaks to the crowd by acknowledging the fact that Brutus is "honorable"; he sarcastically says Caesar was ambitious- but he refused the crown three times. He shows the crowd the cloak Caesar was stabbed in, and shows where each man stabbed Caesar.
"The noble Brutus
Hath told you Caesar was ambitious:
If it were so, it was a grievous fault;
And grievously hath Caesar answer'd it.
Here, under leave of Brutus and the rest,--
For Brutus is an honourable man;
So are they all, all honorable men,--"
"But Brutus says he was ambitious;
And Brutus is an honourable man.
He hath brought many captives home to Rome,
Whose ransoms did the general coffers fill:
Did this in Caesar seem ambitious"
I speak not to disprove what Brutus spoke,
But here I am to speak what I do know.
You all did love him once,--not without cause: