But setting all of that aside (which is a big aside), even if he was confronting them with his camera while armed, the whole bullshit shtick of the second amendment is that being armed should not be a crime much less a death sentence. He did not brandish his weapon or threaten law enforcement in any of the half dozen videos that have been released so far. To be even more clear as a citizen you are allegedly supposed to be protected from summary execution/judgement with or without the possession of a firearm, in many legal circles the possession of a firearm grants you more protections under the law not less.
I suspect a partial answer might be that many are not in their right minds, are under trained, and over motivated by bonus payments and past divorces.