Et tu, Brute?

The quote "Et tu, Brute?" was spoken by Julius Ceasar in one of Shakespere's plays. It is Latin and is translated, "And you, Brutus?" He said this when he found out that all of the city betrayed him and were going to kill him and Brutus, his right hand man, also was part of the betrayal. Why did they want to kill him though?? Can u plzz answer that question and give me any more information about the event. Thankyou!=]