You can say I should join the tin foil hat club, but dammit, I find myself willing to entertain the possibility that there is a sort of informal competition/initiation/bragging rights thing going on. Without three camera, Hollywood-worthy, video and soundtrack, these executioner cops end up on administrative leave with pay, are defended for free by union lawyers, and walk 99.9% of the time. Excellent odds.
Why would they do it? Mix of macho cop culture of control-unto-death, racism, and being brought up on hard-core violent and pornographic video games that desensitize men from a very young age to almost any shred of empathy or even concern for the “other.” It certainly works for the military. Add to that my personal favorite unexamined source of police violence, steroid abuse, et voila! In-group one-up-manship notch on the gun-belt.