A hitman finds himself sharing a booth in a small town diner with the mobster who hired him and his proposed victim. The killer takes a liking to his target and turns down the job, which doesn’t sit well with his employer. That’s when the guns come out and he has to do some fast talking before things turn deadly.
This would make a good short film. Single scene done.
But if you are thinking feature, focus on his main goal after the twist. Then what is stopping him from reaching that goal.
“A hitman decides to save rather than kill his victim which makes them both targets of his boss”.