At first sight, Cassius may well appear to be the villain of the piece. In Act I we see him as cold and cynical, motivated by personal envy of Caesar, instigator of a murderous conspiracy, and manipulator of his more noble friend Brutus. Caesar expresses profound distrust of him, painting a picture of a classic malcontent:
Seldom he smiles, and smiles in such a sort
As if he mocked himself, and scorned his spirit
That could be moved to smile at anything.
Such men as he be never at heart's ease
Whiles they behold a greater than themselves,
And therefore are they very dangerous. (I.ii.204-209)
However, Shakespeare goes on to give a fuller and also more sympathetic picture of the man. Although initially he appears to be controlling Brutus, Brutus soon takes charge, and Cassius proves to be very loyal and respectful to his friend, to the point of letting his own better judgement be overruled by Brutus’s more naïve decisions.
Cassius is also shown to be capable of feeling; in fact he is deeply wounded by the quarrel with Brutus in Act IV, scene iiii. He has a need for personal friendship. Generally speaking, he operates on a personal level, rather than acting from abstract notions of honour and justice like his friend Brutus, and, while we may not always condone his actions, this perhaps makes him a somewhat easier character to understand than Brutus.
Cassius also acquits himself with dignity on the battlefield, choosing to kill himself rather than be humiliated in defeat. Titinius commits suicide in grief over his death, while Brutus pays him an emotional tribute:
The last of all the Romans, fare thee well!
It is impossible that ever Rome
Should breed thy fellow. (V.iii.99-101)
This is significant recognition by the man who is generally held to be the most noble in the play.
Cassius, then, is capable of inspiring loyalty and lasting friendships. Indeed, by the end of the play, he appears quite a different character from the unscrupulous schemer of the opening act. He has lost his earlier blustering confidence and arrogance, he realises the full extent of the failure of the conspiracy which he set in motion, but he accepts his end with a fatalistic calm.
In summary, we can say that Cassius does initially appear to be a rather one-dimensional villain, but this impression of him fades as the play wears on. Shakespeare increasingly chooses to focus on a different side of him, a more vulnerable and human side, thus creating sympathy for him.