That's literally what the police would have done if he didn't confront the police with a gun. You don't need a warrant to knock on someone's door, and before getting one I'd imagine someone would at least need to speak to him.I’m not saying he should get off scotch free. Just arrest him and put him away for a couple of years but do so with a warrant.
The options were either to confront him (and likely kill him), or ignore him. The fact that people are seriously considering the latter speaks volumes about the underlying lack of trust in American state institutions.