Being able to pull the trigger on a human being that wants to kill you requires more than physical preparation. It requires mental conditioning as well. Recognize that no matter how well trained you are, by not being the aggressor you are always the victim. Being the victim means you are in a reactive position and the disregard for law and life that an armed attacker brings to a fight means you are always at a disadvantage. We can compensate for the natural hesitation that comes with being the victim of aggression, but we can never eliminate it. The extension of that hesitation by just a few milliseconds can cost you your life. That means you have no time to waste when reacting to an attacker.