Mind set is the key thing here. Their is a shift from self preservation to self sacrifice. You are choosing to go into the meat grinder. I suggest reading this article on Sheep, Wolves and Sheep dogs to help get your mind right.
http://www.killology.com/sheep_dog.htm
There are several good articles hear some dealing with active shooters. I have met the author and developed a great respect for him. He knows what he is talking about.