Brutus' character is summed up at the end of the play in a speech by Marc Antony:
This was the noblest Roman of them all:
All the conspirators, save only he
Did that they did in envy of great Caesar,
He, only in a general honest thought
And common good to all, made one of them.
His life was gentle, and the elements
So mix'd him that Nature might stand up
And say to all the world: "This was a man!"
The very phrase "this was a man" echoes the "ecce homo" that would have been inscribed on crosses in the churches Shakespeare attended.
Cassius, on the other hand. has a "lean and hungry look," a hunger not for food per se but for power. His defense of the Republic is motivated to a degree by the way in which he himself and other members of the Senate had more power under a monarch than they would have under Julius Caesar as Emperor. While Brutus acts from pure benevolence, and is genuinely sorrowful at the death of Caesar, Cassius is lacking in remorse and more self-interested.
Brutus killed Ceasar from a sense of civic duty,not malice, nor greed nor envy. But Cassius killed Ceasar out of the fear what life under king Ceasar's rule could mean for himand the privileges he has.