Again, I don't know what happened, only what I read here. My only bias is against the threat that was presented.
Here's a hypothetical for you. Let's say you're in your home and happen upon a sleeping burglar with a gun in his lap. When you wake him up he goes for his gun. Are you justified in using deadly force against him? I know, some may claim that the cops weren't at home. That is the big mistake. The cops were legally where they were, performing a legal function, with lawful authority--just like you in your home. If a cop walks up behind someone and puts a bullet in his head, it would be a bad shooting and so would it be if you did that in your home (in most jurisdictions, absent other circumstances).