Well, why does that person need protection? Is it because he's committing an act of violence against whoever he thinks he needs protection from?

No, but ironically, he is committing an invasion, against everybody. To himself he's a disciplined warrior defending his precious family against the forces of chaos. To me he's a poorly-trained, hysterical coward willing to pollute the entire world and fill it with guns and surveillance cameras pointed at ME on behalf of his family about whom I'm not obliged to care in the slightest. God forbid I should be in that neighborhood if I'm black or "acting weird" in accordance with my god-given rights. What are this random guy's criteria and thresholds for threat assignment? With the police, I know what they are or could find out. And ostensibly they're trained for each thing. So yeah, ironically, as little as many people trust the police, they're still way more trustworthy than this guy.

Well, if you watched the news about how many people have felt threatened by a “menacing looking minority” you would understand why “people protecting themselves” is often worrisome.