In the movie El Diablo, Thomas Van Leek, played by Louis Gossett, Jr., is a no-nonsense gunslinger who shot a number of antagonists in the back.  When asked about it, he replies simply, "Because his back was to me."

That is supposed to be a comedy line of course as we know no western hero worth his salt would ever deny his opponent the opportunity to draw first, much less shoot him in the, well...a coward.  You see, much like the Hall Monitor Mentality, the Lone Ranger Mentality has been deeply imbued in the American male psyche.  The Lone Ranger never drew first, never threw the first punch, and my God…never shot a man in the back. 

And many a good man has shed the fullness of his life on the asphalt and dirt because of that ridiculous programming.

Just last weekend we had a suicide bomber in Bavaria.  No, it was not a chemical experiment gone wrong by chemist refugees.  It was a Syrian that had pledged allegiance to the Islamic Jihad.  I am told the bomb was quite sophisticated.  As I suspected, with the increasing push into the west by these people, the war knowledge of how to make better bombs will become more widespread.
I already discussed the COF as the new target for today's threats.  Many seem to have not gotten the ethos of the discussion.
1).  This is NOT a Failure Drill, or a Drug/Body Armor Drill, or a Mozambique Drill.  Here we are specifically and clearly, without euphemisms or verbal deflections, targeting the face/neck area of the bad guy to kill him as quickly as possible.  If that takes one shot, great.  But if it takes fifteen to the face, that is acceptable as well.
2).  America needs to get its head out of the "mugger in the parking lot" paradigm, and it needs to stop reading articles penned by people attempting to sell them legal services. Certainly, there are still muggers in parking lots, but that "crime" paradigm will not work in an active shooter or terrorist event.  As we progress into the decade, whether the traditionalist "defense-minded" trainers like it or not, not all shootings will be "self defense" shootings.  Some by the very nature of the events in discussion will be proactive pistol ambushes designed to take and kill the terrorist unannounced and by surprise. And not only is that perfectly legal, it is praise worthy.
3).  We are discussing PISTOLS, not rifles and not shotguns.  Although rifles and shotguns are far preferable to go to a fight with, I severely doubt that many of you will get into a fight with anything but your pistol.  Understand the distinction - "Go To", and "Get Into".  One is planned and proactive, the other is not.  Unless you go about your day with a Daniel Defense M4 slung over your back, I think what you will have when you face the terrorist is a handgun...hopefully a good one.
Back-of-head-1 Headshot2
Now to the target.  Often the face will not be, well, facing you.  That is a huge blessing as the terrorist will not know specifically that you are there.  So lets consider the side shot and the rear shot. 
From the back, we have an area roughly delineated by the top of the ears across, down from there on each side, and then bottomed out by the base of the neck.  I am stopping at the base of the neck, not because a spine shot will not have a great result, but because a ballistic vest, or a bomb vest may obscure the target. We don't need a degree in anatomy to see what lies just under the surface.
From the side, consider the focal point the ear canal.  But a rectangular box with the ear as the center will give you an idea of what is available.
I seem to have taken a great many words to tell you that it is not only acceptable, but encouraged that you shoot the terrorist/active shooter in the back of the head, without saying a single  thing to him.

And if the Lone Ranger Association wants their stupid white hat back, and Michael Moore doesn't want to be your friend, fear not...I have plenty of black hats for sale at the store, and real men who will call you brother.