At Caesar's funeral, which happens in Act III, Scene 2, Brutus argues that what the conspirators did was necessary. He says that Caesar was trying to get too much power for himself. For this reason, he says, Caesar had to be killed. Killing him was the only way to save Rome.
Brutus says that he hated the idea of having to kill Caesar. He points out that they were friends. He says that he loved Caesar, but he helped to kill Caesar because he loved Rome more.
Because he has done it to save Romans from "slavery," he says, he has no reason to apologize to anyone.