Try to be patient with him, Bill--Luke is trying to use logic.

"Okay, so he wouldn't want to die. What allowed you to kill him, then, if he controls you? I mean, just the fact he didn't expect you to be real and in his universe? Because... it seems like if you had the free will to do what you did--get into his universe, take an action he could not control--you also have the free will to have gone about this a different way."

Does that make sense?, his expectant expression asks.