Why? Only Brutus himself knows exactly what he was thinking when he made this tragic choice. He goes into his own monologue about choosing what is right and what is wrong while standing in his orchard. He debates many things, but in the end his tragic choice is the one for the worst.
Brutus decides to kill Caesar, but he is acting dishonorably in doing so. All Caesar has done is good for Rome. He more than doubled the size of the empire since he became consul. Caesar has not done any bad for Rome, but Brutus and the rest of the conspirators think that he has. And he has done nothing bad at all to anybody but Pompey since he defeated his armies. But in reality, Caesar did not want to kill Pompey, but the Egyptians thought Pompey's head would make a good gift for Caesar.
As Brutus said, "I have not known when his affections sway'd more than his reason. (Shakespeare)." In regular English, that means, "I don't know when he ruled with his affections instead of what was smart." If Caesar had only done what was smart for Rome, why kill the man? Obviously he cared about the general welfare of the Roman people. Caesar cared so much, that he turned down the kingship of Rome three times. If I cared about an empire, and I knew a kingship wouldn't be best, then I wouldn't take it no matter how many times it was offered to me.
Another thing Brutus said was that, "I know no personal cause to spurn at him- If Brutus had no personal reason to after Caesar, then why would he want to kill him? The only truth is that Brutus really did have a reason to kill Caesar. That reason was a personal vendetta that he had from an earlier time in their friendship. Brutus was being "noble" because he is the noblest person in Rome. His relative helped end the last king, so why shouldn't this Brutus do the same.
In reality though, all that was accomplished in Brutus acting foolishly here was he ending up being killed by somebody else in the end of this whole fiasco.