Seems to me that the system works quite well for most of the time.
The police investigate, locate and arrest suspects
The court system determines innocence or guilt - and punishment
The legal system enforces that punishment.
As in any societal group, the police will be made up of many different personality types - ranging from compassionate all the way through through to thug/psychopath.
They also cover the full gamut when it comes to decision-making skills, education and intellect.
When their part in the legal process stops at arrest they are fulfilling their duty which, as I recall, is to uphold the law, protect and serve. And the system falls down when they think that they are entitled to arrest, judge guilt, and administer punishment - which would seem to be the case when they kick a suspect on the ground - ill or not. If one of us kicked a person on the ground we would be arrested and charged. I see no reason why a policeman caught on video should not also be arrested and charged. Given his position of authority and trust, I believe that he should also be terminated from his position.