Cassius and Brutus appear to be on the same side, but with a closer look it's clear that they are character foils. Cassius makes every decision based on what he personally has invested in the issue and what he could have to gain. When he proposes killing Caesar, he says to Brutus that it's not their fault that they are not "as great" in the public eye as Caesar, and that potentially they could be even better than Caesar.
Brutus however, acts only for the greater good. He only agrees to kill Caesar after he has weighed the decision heavily and decided that the act would be in the best interest of Rome as a whole.
