He kills Caesar because he is convinced by Cassius that Caesar is getting too powerful and the power is going to get to his head and he won't care about anyone but himself. So he wants to kill Caesar before this happens. He also kills Caesar because the letters that were faked by Cassius that were supposedly from Roman citizens ask him whether he will bring justice for them by killing Caesar. And since he cares about the people, he wants to listen to their pleas.
