What led Cassius and Brutus to kill Caesar?
Two different motives, I think. Cassius seems pent-up with personal aggression towards Caesar, and real envy of how powerful he has become:
He had a fever when he was in Spain...
Ye gods! It doth amaze me
A man of such a feeble temper should
So get the start of the majestic world
And bear the palm alone.
He persuades Brutus by giving examples of Caesar's weakness, and employing extraordinary similes ("like a colossus...") to emphasise that Caesar has become extremely, extremely powerful. Cassius himself though, is never really presented as having honourable motives. He's serving his own interest, and his own dislike of Caesar: even Caesar himself says that men like Cassius "are never at heart's ease / When they behold a greater than themselves".
Brutus is made to seem much more noble and honourable. He himself claims that it's "not because I loved Caesar less, but because I loved Rome more". Yet Brutus is also hugely self-regarding and Cassius knows where to get him:
Brutus, and Caesar: what should be in that Caesar?
Why should that name be sounded more than yours?
And Brutus' response also refers to Brutus in the third person, as well as emphasising his reputation:
Brutus had rather be a villager
Than to repute himself a son of Rome
Under these hard conditions...
So both are motivated by the good of Rome, and their own personal gain. Traditionally Cassius has been supposed the more self-serving. I think Brutus is.