I was thinking about it and the best argument I can think of he could make would be something like this:
"I was still waking up, I realize now she was drunk as hell and that helps explain it, but she was making these weird noises out there it freaked me the hell out, it was like a loud moan... and that combined with the loud, forceful banging on my door, and still being half-awake, created a situation where I was freaked the hell out. Then, I open the main door and see her there, but you have to understand she had a hoodie up and I couldn't tell her gender, and she became even more animated and insistent once she saw me, and then she made a sudden move which I thought might be her going for a gun... and without even realizing it, I just instinctively pulled the trigger. It was like a reflex reaction... and I feel horrible about it now that I realize her situation and that she didn't have a weapon. But at the time... I really was in fear for my life."
If he were to make that argument and it were to be true, I could sort of see it. I still think he'd *probably* be deserving of a few years in prison, but if he got off scott free and that was his argument, I wouldn't be outraged about it.