Why Castro had to kill Kennedy, legally

I think you're failing to grasp precisely the circumstance that Castro was in. He was facing, literally, daily, highly professional assassination attempts from the CIA, at Kennedy's orders. Hence, there was the chronic threat of imminent harm, in a way that perhaps no one, ever, has been in, before, or since. In order to defend himself, given Kennedy's obsession with killing him, he had to kill Kennedy. You may not care for the argument legally, but, I think most juries would buy it. In fact, I doubt most prosecutors would even want to take the case on, given a neutral jury. Which would mean, not Americans, of course. Interesting legally, don't you think?