Why does Brutus kill himself?

In ancient Rome, it was considered more honorable for a Roman leader to commit suicide rather than face the humiliation of capture. If Brutus were taken prisoner, he would have likely been chained and paraded down the streets of Rome as a trophy, and he would ultimately have been executed for his crimes, so Brutus likely chose suicide to avoid such suffering and shame. Also, since Julius Caesar is based on historical events, Shakespeare simply presents this historical fact in his play.