why should caesar be killed?

Caesar according to Brutus is a very tyranical man.This idea has been put into Brutus' head by Cassius when they meet at the forum in Act1 Scene2.So Brutus has more love and onour for and inthe name of Rome so since he a man who will do almost anything for nobelity and honour Cassius sees this weakness and stikes at this point where Brutus falls into his trap.Later when brutus is sure he is now in charge of the conspirators he thinks that Caesar must be kille before he is crowned(The figure of speech here-Caesar is like a serpent in it's egg which only when it hatches will cause trouble where Caesar when he becomes king only will show his true colour.),thus must Caesar be killed before he is crowned.

If we stay within the parameters of scene one, I'd have to make it simpler than what is stated above, and leave it at the fact that Brutus (goaded on by the other tribunes) sees Caesar as too ambitious. He is convinced by Cassius and others, that Caesar should be killed before being allowed to rule the Roman Empire.