Brutus and the conspirators kill Julius Caesar because they believe he is bent on tyranny and is set to dissolve the senate (and thereby dissolve the republic of Rome). When Cassius speaks to Brutus to convince him that Caesar must be killed before he can be crowned emperor, Brutus acquiesces and compares Caesar to a serpent.
And therefore think him as a serpent's egg,
Which, hatch'd, would as his kind grow mischievous,
And kill him in the shell
The preemptive action proposed by Cassius and agreed to by Brutus and the other conspirators is meant to defend the voting power of the senate and thus protect the republic of Rome against potential tyranny from a single ruler.
In explaining the murder to the crowd, Brutus cites this political motive clearly.
If then that friend demand why Brutus rose against Caesar, this is my answer,--
Not that I loved Caesar less, but that I loved Rome more.
Had you rather Caesar were living, and die all slaves, than that Caesar were dead, to live all freemen?
(The entire section contains 2 answers and 556 words.)