How to remain calm under duress? It takes discipline, experience, and training. Discipline in putting knowledge into action. Regulating your breathing to lower your heart rate. Experience with practice in firearms and combatives as well as visualization and forethought. Training with your firearm and combatives in the event you only have your hands in a moment of truth.
This incident happened today in Arizona. It is a tragedy in every sense of the word. This officer responded with complete professionalism and cauterized the threat. His actions provide excellent lessons-learned for the average citizen and we can only guess how many lives he saved today. Please take a moment to consider the first responder and civilian victims from this tragic incident.

Extraordinary shooting. Am surprised that the officer, given the distance to the actor, did not deploy a long gun (shotgun or rifle/carbine). Perhaps that department doesn't authorize long guns for normal patrol. Luckily, the area was clear of other people, as youn"own" every shot fired..
Take care,