I think the point is that he did/does live in Detroit. If you live there, you know what it's like. You know it can be dangerous. He was a gun owner, someone comes banging on his door or whatever is going on at 430am. You are dead asleep and you hear something like someone is breaking in? What do you do? You have an 81 yr old grandmother in the house to think of as well. If Detroit is as crime ridden as it seems to be, and their police are as unreliable as they seem to be, you probably think you need to defend yourself.
Granted we don't know what happened on the porch. Hell, for all we know, she could have been trying to break in. For all we know, she might have just been trying to break in to call home or just to use the toilet too. And in terms of the shooter, for all we know, he could have thought there might have been 3 or 4 people on the wings of the door waiting to jump him if he let the damsel in distress in the door too.
Bottom line is we need a lot more info to make any sound judgements in this case.