The police officer stamped on the guy's head because a) he thought he had taken or tried to take his gun and b) the officer had just taken a blow to the head from the same man and was panicked.
I think any criticism from anyone who has not been in a situation of life or death split second decision making, they should pipe down and thank their lucky stars they don't have to do such a scary and dangerous job.
More broadly I think Brits have a real problem with policing. On the one hand they expect police officers to run towards danger and act fast where a terrorist incident is suspected - look at how much criticism the anti-terror police came in for not acting on suspicions at Manchester Arena bombing.
On the other hand they expect the same police, in those terrifying moments when for all they know a bomb might be about to go off in the airport, (and they are specifically trained to expect the worst case scenario in action, for OUR safety) to take the time to calm down and think of every outcome or how their action might look to idiots on twitter.
In my country of origin, a family who attacked police in an airport would be full of more holes than swiss cheese about 1.2 seconds later. They wouldn't be hiring lawyers and giving interviews on telly.
The British attitude is unsustanible and no wonder police are quitting their jobs at historically high levels. The Brits will soon see what life is like when there aren't enough police to keep any control.