If I were Cassius, defending the killing of Caesar to the people, I would lay stress on the fact that Caesar was not a fit ruler for them and had therefore to be dealt with. Unlike Brutus, who is far more concerned with political ideals, I would take a much more personal slant on the whole affair. I would point out that Caesar was really a very ordinary, indeed quite weak man who somehow had managed to worm his way to the top, becoming full of pride and conceit as a result, acting in a grand manner and surrounding himself with all manner of flatterers. I would emphasise that having too much power had made him dangerous and I would chide the people for unthinkingly celebrating him when he really was a tyrant who didn't care for them at all. I would ask them if they really wanted to end up being slaves to an inferior man.
In contrast to Brutus, then, I would play up Caesar's personal defects, painting a negative picture of him. Instead of presenting Caesar's killing as a tragic necessity the way Brutus did, I would paint it more as a case of good riddance!