Is Cassius responsible for turning Brutus's thoughts to murder Caesar?
Without the manipulation of Cassius, Brutus would not have thought of murdering Caesar. And most likely there would not have been an assassination at all, because Cassius needed Brutus as a figurehead to recruit the other conspirators. Cassius is not well liked. This is because of his mean, miserly character. Brutus is well liked and respected because of his noble character and because of his lineage. His ancestor was instrumental in driving the Tarquins out of Rome.
Cassius is Brutus' brother-in-law, a fact which gives him easy access to Brutus at any time. Cassius is cunning. He knows how to manipulate Brutus by appealing to his pride in his ancestry and to his genuine concern about the good of Rome. Cassius cares little about the good of Rome, but he cares a great deal about himself. He is afraid he will suffer personally if Caesar gains more power. Caesar might even have him put to death. After talking to Brutus, Cassius reveals himself in his soliloquy in Act 1, Scene 2:
Well, Brutus, thou art noble; yet, I see
Thy honorable metal may be wrought
From that it is disposed; therefore it is meet
That noble minds keep ever with their likes;
For who so firm that cannot be seduced?
Caesar doth bear me hard, but he loves Brutus.
If I were Brutus now and he were Cassius,
He should not humor me.
If Cassius were a good friend of Caesar, as Brutus is, Cassius would never think of assassinating Caesar. Cassius could benefit rather than suffer if Caesar became an autocratic tyrant. This admission is a shocking revelation of Cassius' true character. He thinks only of himself, whereas he knows that Brutus is a true patriot and would sacrifice his own advantageous relationship with Caesar for the good of the Roman people. Cassius consistently works on Brutus' noble character, pride, and patriotism to induce him to betray Caesar.
With Brutus as the leader of the assassination plot, it is easy for Cassius to draw many others in. Cassius learns, however, that once Brutus is committed he is hard to handle. Cassius wanted Brutus mainly for show, as a figurehead. But Brutus takes over the whole assassination plot and then assumes the role of supreme military commander when the two men are forced to flee from Rome after Antony arouses a mutiny. If Brutus and Cassius had defeated Antony and Octavius at the battle of Philippi, Brutus would undoubtedly have demanded to be the dominant member of any new government. In that case, Cassius might have had to start thinking about another assassination.