Maybe he wasn't. Winkelman didn't see anything. He just had someone else's description of what happened.
"I'm not a police officer, but I'm certainly not going to let someone accused of a crime like that go wandering off,"
The guy wasn't "wandering off" until Winkelman went to his house.
If the other guy had shot Winkelman would it have been a "good shooting" since he was being threatened by a man with a gun? Of course Hughes is described as having long hair so that may make it OK.
That's one gun that the guy will probably never see again.
I, too, find a lot to question here--the first thing being that this all came about because of something a 13 yr old kid said!
Certainly, if he knew the guy was in the house, he should have left well enough alone, and waited for the police to handle the situation.