@Balu
If anything, I think this is an example, as with the Michael Brown case, that just because a person is unarmed, it doesn't make them not dangerous or deadly. The officer attempted to use the taser first,
and it was unsuccessful (which happens surprisingly often, hence why so many officers can be reluctant to rely on it, but that's another discussion) and so through a scuffle, the teenager ends up on top striking him.
This isn't like a regular fight between two drunk guys on the street where a guy gets knocked out and the other stumbles away. If someone is willing to go hands on with an officer, either because they're drunk, on drugs, or whatever the reason, it's assumed it won't end well. If that officer is being struck to the point of feeling himself losing consciousness, he damn well better stop that however he can. It might sound excessive to a civilian, or to someone from a country where things are different than the U.S., but if an officer is about to be out cold, he has no way to stop that person from at that point taking his gun and shooting him, or just beating him to death.
Cue all the arguments about not knowing if his gun would be taken.