I believe we explain it quite easily by my post above. His infatuation with the "real Marxism" of Castro and his hope to finally achieve his destiny in Cuba were genuine. The failure of his trip to MC, the collapse of his marriage, and his dead-end job at the TSBD left him in near-desperation. Then Fate seemed to hand him a golden opportunity in the form of JFK's motorcade route. He went to Ruth Paine's on Thursday as a last-ditch effort at reconciliation with Marina. When she rebuffed him, this confirmed that Fate was indeed speaking and his destiny was confirmed. He carried out a near spur-of-the-moment assassination with an unlikely weapon that just happened to be highly successful. I see no reason to make things more complicated than this.
Okay, but that still doesn't add up to me (I'm going in circles here admittedly). If he was driven by his deep hatred of America, his support for Castro and his view that JFK was an enemy of "the revolution", then why wait until the last day to strike back? All of those reasons are deep, substantive, long-standing. The idea that all of them would disappear if Marina had simply agreed to find an apartment and move back together simply doesn't make sense to me. No pasaran, comrades! Unless Marina moves in with me? Some revolutionary he is. And contrast this planning, or lack of it, versus the planning in the Walker attempt. It doesn't add up to me but that may be just my problem.
Just one on MC: He told Marina after returning that he had given up on Castro, that his treatment by the Cuban bureaucrats was just like the Soviet system. Both systems, for him, failed to meet his vision of a true Marxist society and not worth defending. Now, he may have been lying; I think he was. I find it hard to believe that his long time support for Castro ended because some bureaucrats failed to give him his transit visa. That failure means the entire system, "the Revolution", was a lie? No, there's something in MC that, I think, is involved in his act.