Re: What happens if someone from the past shows up...
There's also the reverse with the Federation refusing to allow one of it's citizens to be prosecuted for a crime committed on a planet that would not be a crime on a Federation world. (Justice - TNG). Wesley Crusher committed the crime and Picard refused to accept it. If you had a captain of a lesser moral code than Picard they could basically let the crew run wild wherever you liked. Rape, murder, theft? Not according to the captain.
We can admit that we're killers ... but we're not going to kill today. That's all it takes! Knowing that we're not going to kill - today! - Kirk - A Taste of Armageddon