Not blaming the guy at all but what seems to have happened is that he was shot because, I'm guessing, he tried to defend himself with his gun against a plain-clothed person approaching his car at 3am on the side of the highway. If people didn't think they needed to carry guns to protect themselves, theres a good chance that would not have happened. Of course, ultimately the officer is responsible for shooting the guy and, of course, none of it is on camera.
So, both men had guns. Both were in 'plain-clothes'. Both felt threatened in a different sort of a way. Now, if the guy who shot the other was not indeed a cop, what would you call this incident ?