In Act I the Soothsayer told Ceaser "Beware the Ides of March" because that plays a big part in the play. Also he was right the whole time on when he was going to die.
Act IV: Sc
"To tell thee thou shalt see me at Phillippi" (4:3 In. 280-285)
In Act II Cassius starts a group called Conspirators, and they plan to kill Ceaser. Cassius want to get other people to join but Brutus says there will be to many.
Act V: Sc
"Farewell, good Strato-Ceaser, now be still; I killed not thee with half so good a will" (4:5 In. 45-50)
Ceaser starts to get to ambitious and tells the soothsayer that he hasn't died yet by mocking him. But then the Conspirators come and stabs Ceaser one by one. When Brutus was up he said "Et Tu Brute" because he couldn't believe Brutus would do that to him.
"And men have lost their reason! Bear with me" (3:2 In. 105-110)
When they flee to Phillippi, Cassius storms in on Brutus and starts yelling at him about betraying him. Then Brutus tells Cassuis that Portia dead and Cassius felt really bad and ended the argument. But then Ceaser ghost visits Brutus and said he would see Brutus in Phillippi so they race there.
At the beginning of the battle Cassius and Brutus promised they'd kill themselves if they lost. So they started the battle and Cassius lost so he killed himself. Then Brutus won the first and then fought Antony who won too. But unfortunately Brutus lost so he killed himself.
What I think the theme is there is always payback. And because they killed Ceaser thinking nothing bad would happen but someone wanted payback which cost there lives.