What motives did Cassius have for assassinating Caesar?

Cassius's motivations in killing Caesar are actually quite simplistic. To understand his motivations, it is important to look at two separate passages of the play. When Cassius lures Brutus into becoming an accomplice in the assassination, he cleverly paints Caesar as a power-hungry monarch who must be stopped. He claims that only men like himself and Brutus are able to stop Caesar, cleverly placing the blame for Caesar's rule not on Caesar himself but on Rome's citizens:

Men at some time are masters of their fatesThe fault, dear Brutus, is not in our stars, But in ourselves, that we are underlings

It seems unlikely the Cassius actually sees Caesar as a tyrant. In fact, Cassius simply seems envious of Caesar's rule. He seems to consider Caesar as unworthy of the throne and as an inferior man to himself. In an earlier passage in the play, Cassius recalls an event from his and Caesar's childhood, in which Caesar nearly drowned:

Caesar cried "Help me, Cassius, or I sink!"I, as Aeneas, our great ancestor,Did from the flames of Troy upon his shoulderThe old Anchises bear, so from the waves of TiberDid I the tired Caesar. And this manIs now become a god, and Cassius isA wretched creature and must bend his bodyIf Caesar carelessly but nod on him.

Cassius rescued Caesar from his deadly fate and is now wholly bitter that Caesar serves as the ruler. His motivations are simple and perhaps quite childish. He doesn't think Caesar is a tyrant, and he doesn't necessarily believe that Rome is suffering under Caesar. He simply thinks that he, Cassius, deserves the throne more. He craves the power and is envious of Caesar's success. He remembers the days of their youth in which he mercifully saved Caesar from death, and Cassius is disturbed to realize he must now bow down and worship this former boy who nearly drowned.