Why was Ngo Dinh Diem killed?

The 1st answer to this question is totally wrong. Basically the people who first answered this question is young and know nothing about the Vietnam history especially the Vietnam war. Ngo Dinh Diem was killed by CIA; not only Ngo Dinh Diem but also JFK. Ngo Dinh Diem didn't want US role involved too much in Vietnam because he know that if the American controlled the South Vietnam, North communists ( Ho Chi Minh ) would blame for that reason and brainwashed people more easier. Because of that, CIA did kill Ngo Dinh Diem.