And that would be murder. I have no problem defending your home. Shooting someone after they left is murder.
As a simple rule, this lacks understanding of human nature.
When you get into a firefight, or any kind of life and death struggle, really, your senses tend to go hyper-alert to things that can kill you, while greying out everything else into the background. The guy you're fighting, you're super aware of him, how he's moving, what he might do next. Meanwhile, the walls around you, the doors, the stairs, you only actively notice them if they impinge on your movement.
All that to say, McCullough may not even have been aware, in a conscious sense, that he had followed the intruder back out through his front door. He may have been so focused on the guy's hands, the bottle he held, and what the fella might do next, that he simply didn't register having moved from his living room to the landing. A part of his brain undoubtedly noted it, but not the part that was getting all the processing power at the moment.
If the criminal justice system, our way of judging guilt and innocence, doesn't take that physiology into account, then it is wrong and should be fixed.
In my view, it's not murder if you are still, as far as you can tell, defending yourself, your family, and your belongings. It remains self-defense.
The rest is on the intruder for kicking it off.