What reasons does Brutus give for killing Caesar?

He would be crown'd:

How that might change his nature, there's the question. ...

'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.

...think him as a serpent's egg

Which hatch'd would as his kind grow mischievous,

And kill him in the shell.

In Act II, Scene I, Brutus is in his garden and has decided that Caesar must be killed. His reasons for reaching this conclusion are that Caesar is abusing his power and that has ascended far too quickly.