There is no mitigating factor. He is a weapons instructor who failed to clear his weapon. The only other explanation is that a leprechaun put a round in his weapon.
Unfortunately, people who handle firearms on a daily basis tend to get too comfortable around them. FWIW, I train firefighters rather than police officers, but am acutely aware that 10% of fire service deaths occur during training exercises. The day you don't get a couple of butterflies in your stomach when a cadet does high angle or during a live fire exercise is the day you need to retire. You're going to kill someone.
The thing is, after you make that screw up and let somebody clear a fifth floor window while they're improperly racked onto the rope, all the coulda's shoulda's and woulda's don't make any difference. The guy screwed up. The thing is there are screwups you laugh about and screwups that wake you up in the middle of the night for the rest of your life.
BTW, to all the cop haters on this thread. Next time you're mugged, call a junkie.