I'd call it a heat-of-the-moment thing and no-bill him. It's not like he walked away and came back several minutes later to do it.
Yup, there’s got to be a lot of adrenaline pumping through him at that moment, and when you have to make split second decisions, there’s not much time for second guessing yourself.
Plus, it happened FAST!
It’s not like he was getting in some range time and took his time about it to think things through.
The short amount of time for the first 8 shots IMO, is the adrenaline. The last one to the head? Well, I can’t read the shooter’s mind so will not presume the worst. Perhaps he wanted to make sure. Perhaps he thought it would be more merciful to put the guy out of his misery considering the damage he already did.