Basically, even if the guy has never done anything he was bragging to his friend about... I can essentially see him as a troll. It just so happens that this guy will go out into real life interactions and bully women who stand up for something instead of simply doing it online.
I don't think the guy's actions need much explanation. Especially these days.
The scene of him harassing McDormand? I didn't immediately jump to him being guilty, though I can see why someone else would think that. I mostly saw him as some asshole who heard about the controversy that McDormand started and wanted to bully her into shutting up. Had I gone into this movie assuming it was some standard revenge thriller, I might've thought otherwise. But it was pretty clear to me that McDonagh wasn't making that kind of movie.
In terms of McDonagh telling his story for a certain effect, the scene certainly establishes our desire to want that guy to be the murderer so that he can be punished. And snatching that away from us and Rockwell's character is not an odd warping of the audience. It's the intended effect.
YOU'LL PAY TYRES!
YOU'LL BUY MY EXPLANATION.