Shoot to stop the threat-save your life. (or those of others)  Center mass- dead center- 10X ring plain and simple.

It is NOT the time to get all philosophical about "meaning of life, about to kill someone" etc.. 

It's very basic survival instinct kill or be killed. It's very hard to train the "I'm going to kill some one" thoughts out in that given situation. Part of the constant training.... plus people will always react in high stress situations exactly as they repeatively train for.

Most people do NOT even comprehend the above,,, "shoot to stop the threat-save your life"  They are ignorant of the concept, "if you don't shoot you will die."  They are the same "monday morning quarterbacks,,, "yahbut, why didn't he..??"   Those people in that situation ?? would be a chauk outline on the ground after all was done.

Most people have a piss poor survival instinct.  They even have a name for them......

victims....