Crazy... and desperate.
And, of course, it wasn't a right-winger who shot JFK.
Why did Lee Harvey Oswald kill Kennedy? Here are the Warren Commission's meanderings on the motive for assassination:
It is apparent, however, that Oswald was moved by an overriding hostility to his environment. He does not appear to have been able to establish meaningful relationships with other people."Meaningful relationships" — now there is a 60s catchphrase. I'm glad — but I wonder why — no one uses it anymore.
He was perpetually discontented with the world around him. Long before the assassination he expressed his hatred for American society and acted in protest against it. Oswald's search for what he conceived to be the perfect society was doomed from the start. He sought for himself a place in history — a role as the "great man" who would be recognized as having been in advance of his times. His commitment to Marxism and communism appears to have been another important factor in his motivation. He also had demonstrated a capacity to act decisively and without regard to the consequences when such action would further his aims of the moment. Out of these and the many other factors which may have molded the character of Lee Harvey Oswald there emerged a man capable of assassinating President Kennedy.So it was lefty idealism and disconnection from the practical limits of the real world?