Stupid teenage tweekers meet Crazy Old Man. Everybody loses.
Shooting the boy was probably legally justified. Then it went downhill; he probably crossed the line long before he murdered the girl. I have no idea if he murdered the wounded boy too or not, I haven't followed it that close.
If you hear someone moving around in your house shouldn't your first reaction be to call the police - or even yell "get out I have a gun" at them - rather than setting up an ambush?
Not really. For one thing, the police don't respond to non-emergencies around here, and I don't know if they'd consider a home break-in an emergency or not. Then how long will it take for them to get there? For another thing, there's one of him and two of them, and he may have been afraid of giving away his position, and/or he couldn't safely get to a phone.
Your first reaction is to survive, and we don't have anywhere near enough facts to judge how it started
. It's pretty obvious how it ended, and that enough time passed he can't believably claim a "heat of the moment" thing where he didn't realize the threat was over.