The point is "unarmed teens" kill people all the time and are extremely dangerous. The adult thug tried to take the cops gun and got shot = good shoot.
By definition, unarmed teens never SHOOT and kill people. As far as your last statement, I'm a pretty law and order conservative libertarian willing to give the cops the benefit of the doubt but I don't think this is at all cut and dried. For one thing, we have only the cop's word that this even happened, let alone that his interpretation of any scuffle is correct; since the kid is dead, he can hardly gainsay the cop. There are apparently witnesses who contradict his story though. Also, the kid appears to be big as a tank, which makes one suspect that he would be capable of wrestling away a gun. Assuming there WAS a scuffle, could be as innocuous as the cop gets scared, pulls his gun, kid gets scared, pushes gun away, and runs away. Can't be a good shoot if the cop lied - or is simply wrong.
For another, morally it's gray ground as to at what point shooting him is acceptable. Even assuming he did try to take the cop's gun, does that make him a mortal threat to the public or to the cop? Can't be a good shoot if it's not morally justified.
For a third, legally it may be a bad shoot. Cops get a LOT of latitude, but they are also bound by strict guidelines and standards of behavior simply because they are authorized to use deadly force on our behalf. Can't be a good shoot if it's not legally justified, and in most cases shooting a fleeing suspect in the back is not legal barring some great mitigating factor (in the eyes of a reasonable man) justifying the suspect as a mortal threat to the public or to the cop. (Note that this also has to be within the cop's knowledge.) This far the kid appears to be straight, without any mitigating factors other than he's a teenager, he's very large, he's black, and he doesn't ever seem to smile. None of those things make him a mortal threat to the public or to the cop.
Seems to me there's about a 90% appearance of about as bad a shoot as one can make.