I think it's a lot of adrenaline and muscle memory...you fight like you train. Sometimes the bad guy's still moving or the officer thinks he still moving a posses a threat...it's usually a judgement call as far as how many times you shoot someone. Contrary to popular belief, most officers are not firearms enthusiasts and are not very good shooters. Most officers who are good shooters and enjoy shooting usually move into SWAT or special units where those skills and their knowledge is appreciated and desired. Many times (usually), officers will loose track of how many times they fired their gun and continue to fire until they're empty...then they reload. I always tried to count my rounds...which in a real shooting is nearly impossible...but I always tried and that kept my mind occupied sort of and not thinking/worrying about any return fire. By trying to focus on shot placement and round count and by shooting and practicing a lot...I never got nervous or scared during the shooting. Afterword I would spaze out a little, but during the shoot I was stone cold business. I still have occasional nightmares and reactions to certain noises, colors, or more often smells. I guess it's just part of what happens...my wife and family say I've got a little PTSD, but I don't think what I've been through even compares with what our veterans have been through. I'd be embarrassed to admit I had PTSD...there are so many who have it so much worse.
In answer to your question about double taps...it used to be called "the Mozambique drill". That would be two to the chest, one to the head or the "fail safe drill". We train to automatically fire that way in case the bad guy is wearing body armor or if you miss the vitals you shut off the switch by the head shot. Now they're beginning to train two to the chest and two to the head.
I don't think the officers realize they're still shooting...it's pretty traumatic. The saying goes that if it's good enough to fire once...firing more can't hurt. Because of TV and the movies everyone thinks bad guys just explode when you shoot them...but in real life, they don't...in fact, it's usually pretty hard to even see that you've hit the bad guy. See, folks don't immediately gush blood like in the movies...we're just not that juicy. So when you hit the bad guy it doesn't show up like a big red hole...it's hard to see if you've really hit the guy...so you shoot again and again until your sure their not going to get up and kill you. Don't forget...you're more scared than you ever have been in your entire life and a second prior that bad guy was trying to kill you or someone else...or you wouldn't be shooting in the first place.
Yeah, my heart goes out to all the poor 'traumatized' officers...Milton should consider himself lucky, he doesn't have to live with the memory of getting shot to death...