JMO, but I don't think anything can "prepare you" for the after math of a shooting. Each person will handle it differently, and in their own way. Some may continue on as if nothing has happened, some may seek counselling, either individual or group, or some may turn to religious counselling. In the worst case one may not be able to handle the fact that they took anothers life.
Am I prepared to take anothers life to save my own or a family member? Yes. How will I handle it after it has happened? I don't know, and pray that I never have to find out.