People to try to rationalise an act of pure thuggery like this, because it goes well beyond their day to day experience with people. Seeing a person simply walk up to another person and kick them in the face for no apparent reason is not something the average person sees every day, so it is natural to speculate on what this act might have been a reaction to.
However, we should keep in mind that there are some very nasty people out there who commit this sort of violence simply because they enjoy it. Unfortunately, some of them manage to find their way onto sports fields. We've all seen them and at the highest level too, and we all know who they are... they have been discussed on this board frequently.
When I first saw this video, it very much reminded me of this incident from an RL match in England a couple of years ago...
https://www.youtube.com/watch?v=BZ5NlD9RsyQ
... IMO, this is an example of a player with a nasty, violent streak (he had form and had been sent off for punching on previous occasions) completely overreacting to a minor provocation. A few seconds earlier, he pushed White 6 over. A second or two later, White 6 responded by pushing him back. Most players would have left it there, but Purple 10's reaction was to deck White 6 with a punch and then king hit him in the face while he's lying half-conscious on the ground. Players like this have no place in the game and ought to be banned permanently.