What reason does Brutus give in his soliloquy for killing Caesar in Julius Caesar?
Brutus thinks that once Caesar is given power, he will become corrupt. To prevent him from bringing harm to Rome, Brutus is wrestling with the idea that he should kill him.
Brutus actually has no specific evidence to think that Caesar will become a bad ruler. He says,
"...for my part I know no personal cause to spurn at him" (II,i,11-12),
yet speaking generally, he is afraid that power
"...might change his nature, there's the question" (II,i,13).
Brutus believes that if Caesar is crowned, that will put in his hands the capability
"That at his will he may do danger with...Th' abuse of greatness is when it disjoins remorse from power...And to speak truth of Caesar, I have not known when his affections swayed more than his reason" (II,i,16-21).
Because of Caesar's personality, and in particular his tendency to be completely unemotional while relying solely on reason, Brutus feels that there is a great possiblity that he will let power go to his head and do damage to Rome. Remembering situations where those who have climbed the ladder of ambition have turned their backs on their more noble inclinations once they have achieved their lofty positions, Brutus reflects,
"So Caesar may. Then, lest he may, prevent" (II,i,27-28).
To protect Rome from what Caesar might do once he is in power, Brutus resolves to kill him, like "a serpent's egg" (II,i,32) while still "in the shell" (II,i,34).
Well, ominously, he starts without giving any reasons, but with a firm conclusion:
It must be by his death
The rest of the speech then works to justify that conclusion - that Caesar's death is the only way to solve the problem in hand. But here's Brutus' argument:
He would be crown'd:
How that might change his nature, there's the question.
Caesar wants the crown. And that might change the way he behaves.
... '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. So Caesar may;
Then, lest he may, prevent.
Humbleness and lowliness is like a ladder, which "Ambition" climbs up. When ambition has got what it wants (and got to the top of the ladder), it forgets how it got there, and, rather than look at the ladder, it looks into the clouds. And Ambition "scorns" the way it got where it's got. Caesar could do the same. And therefore, prevent that happening.
...think him as a serpent's egg
Which hatch'd would as his kind grow mischievous,
And kill him in the shell.
Caesar, therefore, is not dangerous at the moment. But he's like a serpent's egg: one day he will become dangerous, because that's what serpents all are. So better to kill Caesar now before he becomes a full-grown danger.
I don't think it's the world's best argument. But I hope it helps!
he said that caesar was soon to become a tyrant and treat the as slaves so he and the other consprators killed him in the forum