According to Rory Miller, author of Facing Violence, everyone freezes (or hesitates) for at least a split second when attacked. He devotes an entire chapter to it. Training and awareness can help a person break out of the freeze in order to do what is necessary. If I'm ever attacked to the point where I need to call on deadly (and all attacks certainly do not warrant deadly force) I hope to use that split second of freeze time to make absolutely sure I have no recourse other than deadly force.
I've been in altercations before and had chances to seriously injure my opponent if I did what I trained to do. I've always held back, because it wasn't necessary -- I was able to end the threat without severely injuring the person. I hope I would hesitate at least a little before shooting another human being. I would venture to say that anybody who would shoot another human being without any hesitation whatsoever has a problem. Of course there is always that point of no return, but it's different for everyone.