Cassius wants what's best for Rome, and isn't plotting a conspiracy against Caesar for a personal grudge.
Why, man, he doth bestride the narrow world
Like a Colossus, and we petty men
Walk under his huge legs and peep about
To find ourselves dishonorable graves (I.iii.135-138).
In the passage above, Cassius tells Brutus how the common public enables Caesar to be ruthless to them. He shows his concern for the Romans and questions why they allow Caesar to walk all over them. He seems to think that the Romans might have some contribution to Caesar's arrogance. Also when Casca tells Cassius that the senators want Caesar to become king, at this, Cassius threatens to kill himself with a dagger, "But life, being weary of these worldly bars, / Never lacks power to dismiss itself" (I.iii.96-97). This suggests how strongly Cassius feels against Caesar. By saying this he tries to prove to Casca how much self respect he has and would never be willing to bow before a man like Caesar, in fact he would much rather kill himself beforehand.
Caesar is well known to have a reputation of a tyrant and a weak leader. This is why Cassius believes he is not fit to rule Rome. Caesar has disabilities that make him physically weak as well. For example, Caesar (addressing to Antony) says, "Come on my right hand, for this ear is deaf, / And tell me truly what thou think'st of him" (I.ii.213-214).Besides, his inability to hear properly, he also has epilepsy which is revealed when he collapses in front of the crowd during a festival. Cassius also despises of Caesar because of his ruthless