Of course they are human, but by selecting the role have to put the public safety above their own and control their reactions to be in control at all times, and manage fear so it doesn't affect judgement.
Agreed they can fight back under attack - but only using approved techniques, and of course must stop using inappropriate force once not under attack. At the time of the kick the officer was not under attack by the person on the ground, so there is no relevance to your point.
Any events leading up to the kick are irrelevant - it wouldn't have mattered if the evil little shit had murdered other officers and shot at him, if he no longer had a gun and was on the ground, a kick to the face is never valid - next you'll be justifying he could shoot someone who had surrendered.
This officer, along with those who strip-searched young woman have shown GM police to be out of control and can no longer command the trust of the public. Any good police officers should come forward immediately and disown the actions of these colleagues (all of which are complicit) - otherwise, they are all as bad as we have seen.
You can't have it both ways - Tony Martin wasn't a police officer so had much more right to act out of fear, but the police decided otherwise.