Criminals, by definition, commit crime. That means they are, in general, not very nice and have less control than the general population.
Police are meant to be a professional force. This means that they shouldn’t commit crime. They are given help here by training, defensive armour and an array of weapons, as well as being able to call for assistance.
Kicking someone’s head is a crime. It really cannot be argued that it is the minimum force necessary to restrain someone. Heads are delicate and, once the brain is damaged, it doesn’t repair well. It is easy to kill someone by kicking them in the head.
I really hope the above isn’t too complex or controversial for some who hanker after the days when police could get away with giving someone a good kicking.
There is zero excuse for a policeman to kick or stamp on someone’s head-ever. And, as police are meant to help uphold the law, they should be punished at least as harshly as a civilian if they break it.