Second, cops are trained in a similar manner and that further weeds out those that don't fit in. Both of these are similar to the military with pre-induction testing and then boot camp.
Finally, most cops see a lot of bad things in their careers. Most of those bad things are the result of stupid people doing stupid things and often the ones who get injured are innocent. A guy gets drunk and drives his car and crashes into a van with a mom and kids. Mom and kids are at best scared and at worst killed and mangled. That's hard to deal with and it makes people angry. Child abuse, some three year old beaten with an electrical cord, or child neglect a three year old left alone at home while mom goes dancing; or a baby left alone in a car on a hot day. Cop mentality, it does not have to be a bad thing; that's what the SGT Says.