Caesar didn't understand the fact that the soothsayer was trying to warn him about the Ides of March. Caesar has been so stuck on himself, he doesn't think about all the other possibilities that could go on or think about his people in the town. This is when Cassius and Brutus were planning to stab him multiple times to get rid of him for Brutus to be king. I quote Cassius said "The fault, dear Brutus, that we are underlings."
Act IV: Sc III
In this scene all the conspirators went to Brutus's place, and he has not slept for awhile, he laid awake thinking about the Ides of March. As they entered and stayed they talked about their plan to kill Caesar, as they did so they all talked about killing Antony, but Brutus does not want Antony killed, because he thought Antony was a good guy that he wouldn't cause harm after he had died. Although they had to keep him away from Caesar so he does not get in the middle of the mess. Brutus says "If he loves Caesar, all that he can do is to himself-take thought, and die for Caesar."
Act V: Sc I
In this scene the conspirators get Caesar alone, well with all the towns people, Brutus, Cassius, Casca, and some others stab Caesar and as he's wounded and about to fall, Caesar says "Et tu, Brute?" Then fall, Caesar." As he was dying he felt betrayed that Brutus was also one of the conspiritors to stab him of all the towns people it was Brutus and some others.
Later in this scene Brutus sees Caesar's ghost and it tells him to see him at Philippi, and to them it means death. Caesar came back to avenge his death to tell Brutus that he will basically die in Philippi. Caesar even in the after-life still feels betrayed and wants to see Brutus dead from betraying him.
In this scene they are getting their soldiers ready for the Ides of March, Antony and Brutus are about to fight until the death in Philippi. The Messenger of battle says "Prepare you, generals, The enemy comes on in gallant show; Their bloody sign of battle is hung out, And something to be done immediately."
The theme of Julius Caesar is Brutus is worried about Caesar's way of leading and ruling the town of Rome, and also Brutus is jealous of Caesar, because of all the power he has. So the only way Brutus thinks is killing Caesar. Jealousy is the root of evil that can cause a man or woman to do unthinkable and crazy things.