The argument was, and always has been, that there is no context in which it's acceptable for the police the kick someone in the head once they've been neutralised. Some context has been provided but it doesn't change anything. They guy doesn't even look like he's properly come around after being tasered when the officer kicks him. His brother is also sat with his hands in his head at this point. The situation is under control.
The police have a right to arrest and charge the suspects. They dont have a right to undertake extra judicial beatings. Ever. And if they do do that, your opinion might well be that the suspects "deserved it" but it doesn't change the fact that the officer will be investigated and possibly prosecuted for it.
The suspects should also be prosecuted for their part in it. Nobody comes out of it looking good.