Given the circumstances, you made a good (ethical) call on letting an intoxicated man not pay for his bad judgment call with ETERNITY. Taking someone's life is a big responsibility. Think about the sum of your own life experiences--how old you are, all the friends, family members, and loved ones who know you, everything you've ever done, and put it on somebody else, and then imagine it's snuffed out in the blink of an eye. If that guy was 40 years old, you could have ended 40 years of life experiences, all because he had too much to drink and got confrontational with the wrong person. However, had you fired your weapon, I don't think you'd have been in the wrong in the eyes of a jury. It'd be a pain in the ass to have to go back to court for a civil case from the family suing, though, and wind up being found "in the wrong", because a sympathetic jury thought his drunkenness excused his actions, but I digress.
Were it me, I probably would have fired, for a few reasons. I find the idea of being stabbed with a screwdriver more disturbing than if he were a drunk armed with a gun and shot at me (which would have had me acting in fear for my life, either way, but I REALLY don't want to be stabbed with a screwdriver!). I imagine you were wearing your vest, which, depending on the ballistic rating (3A is typical for most, which is good against many handguns, but not effective against stab attacks.), may or may not have been helpful. Also, 15 feet may seem to some like a great distance, but you'd be amazed at how quickly someone can close a distance of 15 feet, even if intoxicated, Up until he fell, you were definitely in danger. Also, taking into consideration what I previously said, about ending someone's life in the blink of an eye, I'd have thought that this guy had no regard for my life, so he made up his mind that it was either my life or his. I wouldn't let it be mine, because I'll be damned if some drunk guy kept me from going home to my wife and son at the end of the day.
One way or the other, I'm glad to see that you're alive, and nobody's life was ended.