Kyo, you're letting your age show through here. Buck up.
He screwed himself. The fight was over, the perps had split and the remaining one, unarmed from the start, was lying incapacitated on the floor.
This is a great example of what not to do. I would like to think most of us wouldn't need this example of what not to do. You don't walk back inside, walk past a downed perp, walk over to a locked drawer, unlock it, retrieve your other gun, then walk over to the downed perp and empty it into him at close range. That's not self defense, that's an execution.
He had prepared extensively for another robbery, and I think he had thought over what he would do so much that he just went overboard neutralizing the threat. He seemed to be operating under the delusion that it was OK for him to do so. He very calmly finished the kid off, then promptly called the police. Very business like. I'd say he was gravely uneducated and ignorant of what he could and couldn't do. I'd venture the guess that perhaps he was thinking an armed robbery = a right to terminated anyone evolved. Unfortunately he was either uneducated on when his right to use deadly force stopped, or he was flustered and steamrolled right into what appears to have been a cold blooded murder.
It's an unfortunate event, as even the DA pointed out, if he just hadn't done that one last act he would have been fine. He stepped over a line. What's scary is any one of us could find ourselves in a similar place.
I have empathy for him, when these things go down everything hits the fan. Things speed up, things slow down, the world turns upside down and spins all around, it's ****ty.
It looked like his field of vision was somewhat obstructed from his location, he saw two masked robbers, a gun, they were screaming at him, it's reasonable for him to assume they were both armed, and once he started shooting it's going to be hard to tell if one or both are shooting or in this case, if you're just shooting yourself. It's possible what he did was just a terrible misjudgment made in the heat of the moment. That's no excuse for what he did though.
You have to educate yourself and make certain you know your laws and what you can and cannot do to defend yourself, and you can't loose control like that and murder someone, even out of ignorance or heat of the moment. Something in your head should click when you're looking down at an incapacitated person with a gun in your hand, if you go ahead and empty your weapon into them then you have some issues. Not only that, but you will have some issues, legal ones. The law doesn't care if you know it or not, saying you're sorry you didn't know won't cut it, saying I'm sorry I thought I was following the law won't cut it. Not on this scale at least.