The homeowner killed him (justifiably) based on an empty threat. While the homeowner (legitimately and justifiably) thought that his life was in danger, if he hadn't shot the guy, what would have likely happened would be that he would grab his ATM card and keys, start to drive to the bank, the wife would call the cops, and the cops would ambush the criminal when they got to the ATM.
A potentially dangerous situation, for sure, but the couples' lives weren't in immediate danger.