What factors seem to be shaping Brutus’ conscience when he decides that Caesar must die in Julius Caesar?

Brutus argues that Caesar has done nothing wrong yet and is not a bad person, but he is ambitious and ambition is dangerous.

Brutus knows Caesar very well.  He has been like a father to Brutus, whose own father died when he was young.  Brutus does not want to kill Caesar, but he worries about the fate of Rome.  After the conspirators ask him to join them, he ponders the assassination in a soliloquy.

Brutus...

Brutus argues that Caesar has done nothing wrong yet and is not a bad person, but he is ambitious and ambition is dangerous.

Brutus knows Caesar very well.  He has been like a father to Brutus, whose own father died when he was young.  Brutus does not want to kill Caesar, but he worries about the fate of Rome.  After the conspirators ask him to join them, he ponders the assassination in a soliloquy.

Brutus begins by suggesting that although he personally does not have a reason to hate Caesar, he worries that Caesar being crowned king would change him.

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. (Act 2, Scene 1)

Brutus worries about the “abuse of greatness” and fears that Caesar will turn into a tyrant because he is already ambitious.

But 'tis a common proof,
That lowliness is young ambition's ladder,
Whereto the climber-upward turns his face;
But when he once attains the upmost round.
He then unto the ladder turns his back,
Looks in the clouds, scorning the base degrees
By which he did ascend.  (Act 2, Scene 1)

This is the main argument against Caesar. The conspirators believe that Caesar has taken too much power upon himself by being dictator, and that he will leverage the power he has to pre-empt the senate and declare himself king.  Once king, they fear that the Roman republic will be dead, and the senate with it.

Brutus argues that the only thing to do is kill Caesar before he has a chance to do this.  He describes Caesar as a baby snake, and says the only way to deal with him would be to kill him in his shell.  With this, Brutus is committed to killing Caesar in his mind.  He believes that doing so is the only way to save Rome.