The most prominent explanation being that he was stopping an active shooter. But even if we accept that story, it completely undercuts the effective argument of “but he just had a skateboard and flinched!”
I think everyone in this situation thought they were doing the noble thing. It’s just easier to process if we assume one person is a bad guy, and the other person was acting nobly and rationally in pursuit of some higher purpose, rather than accepting the messy truth.