Ehh... looked to me like almost a Hollywood-esque type shot to the head TBH. Gun was kind of cocked to the side, from a downward position, and flush against the skull. I only watched it once, because it was pretty brutal.
As for the difference between manslaughter and murder, it has a lot to do with intent, malice, and culpability. So you can kill someone driving recklessly (speeding, swerving lanes) and be charged with manslaughter, because your intent wasn't necessarily to kill someone, and there was little to no malice. Whereas, you press a gun against someone's temple and squeeze the trigger, there's a clear intent to kill and certainly some malice.
Let's also not forget, as I'm sure there will be some who will try to mitigate this execution in some way; there are two other cops who were more than willing to corroborate the other's story, and had it not been for the body cam, I'm quite certain these charges wouldn't have come.