If we're putting Trump's comment here in context, I'd say he was pretty clearly going for a goof. He was expecting a big ol' laugh after his comment (and there were a few) but it fell mostly flat.

Was he genuinely willing his supporters to go out and shoot Hillary? Or was he cracking a funny? Considering his sense of humour (e.g. Trump mocking that disabled performer), I bet he was trying to crack another funny. He's an idiot, but I don't he's legitimately calling for his supporters to kill someone.