As someone who both had a lover who tried to kill thousands of people and being someone who tried to destroy an entire planet in my dreams, the question you should be asking is if the guy you're into has the capacity to do those things here. This is a different life than those we dream of, and we do not necessarily become what we dream. We have a choice, and if this guy is set against following the path laid out in his dreams, then that is something.
He may need to prove himself to your best friend, for obvious reasons, but if he is dedicated to not following the path of his dreams, he deserves a chance.