1) What is the significance of Caesar's dying words, "Et tu, Brute? Then fall, Caesar!"?
The conspirators gather around Caesar and he sees his trusted friend Brutus among them. Stunned that Brutus is among his assassins, Caesar cries out, "and you too, Brutus?" This famous line is important because it sets Brutus apart from the other conspirators. There is no doubt that Brutus's self-serving and ambitious accomplices have committed an indefensible act, but with Caesar's final utterance we recognize that the self-sacrificing and noble Brutus has perpetrated the same heinous crime – his motivation is rendered immaterial. For this moment, Brutus the idealist becomes Brutus the murderer.
2) Who turns the people of Rome against Brutus?
After Brutus addresses the Plebeians, successfully assuring them that Caesar's murder was necessary to preserve their freedoms (3.2.13-37), Antony delivers his cleverly crafted speech in defense of Caesar. While making sure not to condemn Brutus and the conspirators, he argues that Caesar had no plan to turn Rome into a dictatorship. He reminds the crowd that Caesar was offered a "kingly crown" (3.2.102) three times and refused each time.