We are not talking about other situations, we are discussing what is in this video. What happened in the moments before is to a large degree, irrelevant. The man was on the floor, face down. The officer then approaches him, stamps on his head and then kicks him in the head. To offer a comparison is like saying in the theatre of war, you and the enemy are shooting at each other but then they surrender. However, as they were shooting at you 20 minutes ago you're entitled to shoot them.It absolutely could be reasonable if there was a threat to someone's life and the law is quite clear on that. I'm absolutely not saying that was the case here but I witnessed far more serious uses of force than that which were completely justified in a court of law.
I once had to deal with a guy trying to strangle his mother, he was on top of her at the bottom of a flight of stairs and he wouldn't let go or stop and she was losing consciousness. Long before the days of tasers, I pepper sprayed him to no effect and ended up hitting him with my Police baton at least 8 or 9 times across his naked torso, back and arms, I was running out.of options and was just about to smash him In the head with the baton to stop him killing her when thankfully he gave up. So yeah there totally could be many situations in which kicking someone to the head could be justified.