Probably because he wanted power (to be emperor).
He is going to write him letters in different handwriting to convince him the people want Brutus to rule.
read the book and you will find out dummy hahahah It's a play. Not a book. Dummy.
Cassius!
Um, if i remember correctly Cassius did something with implanting letters saying that they where concerned that Caesar was trying to be there king/ruler; they where worried that the democracy would...
HOTD: Overkill - Boss - Brutus