Earlier quoted context omitted.
These things are still pretty rare. Unfortunately, once you start raiding barber shops and pool halls with so much heat something like this is bound to happen. I still find it to be an honorable job with honest guys that put their lives on the line; however, statistically there's always going to be some hot-headed trigger-happy guy involved. Plus even if they're not on the force they're in some neighborhood as the un…
Give someone assault weapons, train them to believe that they are fighting for good, and remove all legal and moral contraints and watch how quick they turn into a "bad apple". It's not a personnel problem, it's a structural problem. See Stanford Prison Experiment, etc.
Relevant SMBC http://www.smbc-comics.com/index.php?db=comics&id=3025#comic