Heard a couple of black cops interviewed on the radio.
Their take?
Cops are taught to stay one step ahead on the aggression curve, trying to negate the threat. They are also people, who get amped up by high speed chase, foot chases and wrestling with a suspect.
Both cops agreed that you are taught to never, ever, never kick or strike a person in the head. Actually, they agreed you shouldn't kick a suspect. Blows with a baton or ASP are ok to use on a fighting suspect, but strikes should be confined to muscles, the more muscular the area, the better.
Both thought some of the post arrest banter was pure stupid.
Something else was brought out...If these cops normally worked this shift together, it leads to a group mentality of us against the world, on steroids. To combat this, you need good shift supervision and an occasional rotation of shift members.
So...
Adrenaline rush.
Bad training or bad adherence to training.
Bad group mentality
Both cops stated this was not a response based on race.