To be honest, I suspect the police officer could be in trouble here, the public have subdued the guy and then the police have dragged them off and double tapped him.
I can't see him being anything but cleared.
Couldn't taze him - Was wearing a bomb vest to the best of their knowledge. You really don't want to be pulsing high voltage electricity into what could be unstable homemade explosives.
By the looks of it, he was on the ground surrounded by officers. They'll be wanting him to stay there. He tried to get up... Wrong move.
There will be an investigation, some newspapers will praise the officer, others will say he's the spawn of satan who once got drunk at a christmas party and mooned a young woman and is all that is wrong with the world.