Julius Caesar by William Shakespeare is based on an historical event, the assassination of Julius Caesar by a group of conspirators who wished to restore the Roman Republic, rather than seeing Rome turn into an Empire. Because the rebels have agreed to act in concert, they all stab Caesar so that they will all share equally in responsibility for his death.
Brutus is torn between personal friendship for Caesar, who was his mentor, and his moral commitment to a more fair form of government. He reasons that it is precisely because Julius Caesar is a decent person and good ruler that he is most likely to succeed in establishing a despotic form of government and thus needs to be killed, despite being a friend.
When the conspirators stab Caesar, Brutus is last and Caesar utters the famous phrase "Et tu, Brute" (Latin meaning "And you also, Brutus"), indicating that he is shocked and saddened by seeing his friend and protege stab him as well as people he knew to be enemies.