"Honey, there's someone in the garage!"
"Really? You sure? Call the police, I'm gonna go shoot them!"
Yep, if you know your garage is locked, which it was, that means whoever is in there is an intruder, with the intent to cause you or your property harm, the kid was infact there to rob him, therefore his assumption that the kid was there to cause him harm was accurate, and justified, in what other scenario would someone accidentally end up in your locked garage?
