"This is the dude who assassinated JFK."
"If you got a question, just shoot!"
Listen up, buttercup. This joke is about JFK, the president who got his brain splattered all over the back of a convertible. The joke is that the dude who offed him is saying "If you got a question, just shoot!" like he's inviting you to ask him anything, but also like he's gonna cap you if you do. Get it? Probably not, since you're slower than a herd of turtles stampeding through peanut butter.