"He took a human life once."
I read your story and am not sure "human" applies. And in any case there's a reason why there are self-defense laws on the books. It is very human and healthy of you to have some regret and even some shock at what you were capable of - but you only survived to this point of comfortable reflection because of it.
Riddle: What do you call someone who will do anything to stay alive?
Answer: You call them "alive."
"Don't think it hasn't been a little slice of Heaven just because it hasn't!" --Bugs Bunny