Why did Oswald kill JFK?
I posit that the little prick was a true Marxist, that he was fed up with the American and Soviet systems, and that he decided to take matters in his own hands to "advance history".
-- MWT 
PS The fact that he wasn't getting any from Marina might have had something to do with it, too, as well as the possibility that he had been trained/programmed in the USSR to do the job ...
I think that's essentially accurate: that is, his motive was probably a mix of political and personal reasons or grievances. He expressed numerous times his deep dislike of America (even when he was a teenager), of the economic and political systems that he thought were "slave" systems. He said the same thing about the Soviet Union, that it too was a "slave" system. He said he detested both governments and was opposed to both. He also wanted to be, according to Marina and a few others, a great figure, a noted person in history. And I do think he disliked JFK for his (Kennedy's) policies towards Castro and Cuba. So we have this confluence of personal reasons - fame, notoriety, making history - and political - killing the leader of the nation he intensely disliked.
But then we have the question of why did he essentially wait until the last second to shoot JFK? About ten days before the assassination he goes to FBI headquarters - in person! - to complain about what he believed was harassment by them. That's an odd act for someone planning on shooting JFK.
Okay, so at that point he didn't know JFK was coming to Dallas. Or whether he would have an opportunity to kill JFK. Why did he wait until the day before the visit to retrieve his rifle and, essentially, plan things? It all seems so rushed, so last second, so spur of the moment. That's not evidence to me of someone with a deep dislike for Kennedy or the US. That's someone acting hastily, suddenly.
So we have this seeming contradiction: on the one hand we have these deeper political and personal reasons for him to act and on the other we have him acting "late", at the last second, with little planning beforehand. It's a puzzle to me.
