What are Cassius's negative qualities?
My heart is thirsty for that noble pledge. Fill, Lucius, till the wine o'erswell the cup; I cannot drink too much of Brutus' love (Act IV, Scene 3, lines 179-181).
Shakespeare depicts Cassius as a miser in various subtle ways throughout the play. Caesar tells Antony that Cassius has a lean and hungry look. When Cassius invites Casca to supper and then to dinner in Act I, Scene 2, Casca is obviously reluctant to accept but finally says:
Ay, if I be alive, and your mind hold, and your dinner worth the eating.
Casca has known Cassius all his life. Perhaps he has eaten at Cassius's home several times in the past and knows what kind of meal to expect. When Casca turns him down for supper, Cassius ups the ante by inviting him to dinner, which is a big meal served in mid-afternoon. Evidently Cassius thinks that Casca can be lured by a more elaborate meal—but Casca just doesn't like Cassius and is deliberately rude to him when he grudgingly accepts his second invitation. Note how Casca says "and your dinner be worth the eating." He doesn't expect much from Cassius's table.
The big argument with Brutus in Act IV is over money. Brutus sent to Cassius for gold he needed to pay his soldiers, and Cassius sent back a note refusing him. After the two men have reconciled and shared a bowl of Brutus's wine, Brutus still doesn't get the gold. This might contribute to the loss of the battle at Philippi, as the soldiers expected to be paid for risking their lives.
Brutus is not a good judge of human nature. He should never have formed such a close friendship with the selfish, stingy, greedy Cassius. Brutus lost everything, including his wife Portia and his own life, because he let himself be manipulated by Cassius. Brutus had nothing to gain by killing Caesar and everything to lose. Cassius, by contrast, had a lot to lose because he knew that Caesar hated him—and more power for Caesar would spell trouble for Cassius.