In Julius Caesar, how does Cassius kill himself prematurely? Why does he do this?
While Cassius has, indeed, been most envious of Caesar, having hungered to assassinate the ruler of Rome because he himself is "lean and hungry" and not for the noble reasons held by Brutus, he certainly has been able to think far more tactically than the others in his camp. When, for instance, he and Brutus meet in Act IV, they argue about one soldier accused of bribes that Cassius has written in defense of; later, they discuss their battle plans against the triumvir of Marc Antony, Lepidus, and Octavius Caesar. Brutus wants to march to Philippi, but Cassius insists that they remain where they are and have the other armies come to them because their army will be exhausted if they march to Philippi; in addition, if the other army marches, they will add to their numbers because the people support them:
The enemy, marching along by them,
Come on refreshed, new-added and encouraged;
From which advantage shall we cut him of
If at Philippi we do face him there,
These people at our backs. (4.3.232-236)
But, Brutus insists that they will cut off the followers if they deny them, saying words similar to those of Cassius:
There is a tide in the affairs of men
Which, taken at the flood, leans on to fortune
Of course, the early words of Cassius prove to be true, and the forces of Brutus and Cassius are defeated. When Cassius realizes that he has been defeated, he commits suicide rather than suffer the ignominy of surrendering to his enemies. This act is a practice of Romans.