Why was Brutus conflicted in Julius Caesar?

Brutus is conflicted about killing Caesar because he hasn’t done anything yet, but he probably will.

Brutus is approached by Cassius, another senator, who asks him how he feels about Julius Caesar.  Cassius wants to make sure that Brutus feels the same way that he and his group do.  They feel that Julius Caesar is overreaching in his role as dictator of Rome and has to be stopped.  They think that he is arrogant and aggressive, and they want to see if Brutus does too.

BRUTUS

What means this shouting? I do fear, the people
Choose Caesar for their king.

CASSIUS

Ay, do you fear it?
Then must I think you would not have it so.

BRUTUS

I would not, Cassius; yet I love him well. (Act 1, Scene 2) 

Brutus tells Cassius that he would not approve if the people made Caesar their king.  He doesn’t think that Rome should have a king any more than Cassius does.  He also believes that Caesar has started down a dangerous road in his war with Pompey.  Thus, Brutus joins the conspiracy to assassinate Julius Caesar. 

The night before the assassination, Brutus has a conversation with himself in a soliloquy in which he ponders the reasons why Caesar has to die.

It must be by his death: and for my part,
I know no personal cause to spurn at him,
But for the general. He would be crown'd:
How that might change his nature, there's the question.
It is the bright day that brings forth the adder (Act 2, Scene 1) 

Brutus says that he has no personal reason to dislike Caesar, but for the general reasons that he is ambitious and he might be dangerous if he is crowned king.  Brutus says that Caesar has never done anything wrong yet, and never done anything against him.  He just fears what he will do in the future. 

The reason the other conspirators wanted Brutus to join them in the first place was because of his reputation.  Brutus has an old honorable name, and he tries to live up to it.  He feels like he is betraying Caesar by being involved in this conspiracy, but as he tells the people in his speech after Caesar’s death, as much as he loves Caesar, he loves Rome more.  He feels that he has to kill Caesar for the good of Rome.