"As seen here, Caesar is being murdered by his "friends." This may lead to conclude that the theme is "don't trust anyone, even your friends." However, this is not the case.
Later in the play, Cassius commits suicide because he is losing the battle with Mark-Antony. He stabs himself with the blade used to kill Caesar. Cassius then states as the blade is in his chest, "Caesar, thou art revenged."
After Cassius dies, Brutus does the same thing and commits suicide. So the main theme is "if you cause death, it will always come back to get you." This can be seen when Brutus and Cassius die and they both say something involving Caesar being avenged.