Not sure if this qualifies as a "that guy" story either but here goes.

I was GMing a game of Hunter: The Vigil and the group consisted of two guys who had played RPGs before and two who were new to the whole thing. They were a team of federal investigators looking into a string of murders and they were doing quite well in following the breadcrumbs.

One of the newbies, "Agent Moore", was getting really into everything and so was "Dr. West" but West was being a little too enthusiastic about things and started to assert himself over the rest of the group, often shafting Moore in the process and killing his spirit to play over the course of the session.

It was starting to upset me so when they finally tracked the murderer down, I had manufactured a situation in which Moore was the only one with a gun and therefore the only one who should go into where the murderer was. So Moore goes in, catches the murderer off-guard and begins a social combat with him. He was actually making the rolls and talking this guy down.

So West decides that he's tired of waiting and proceeds to fail sneak checks up to the house. I roll with it, hoping to cause a bit more tension before Moore managed to make the final check. West, however, decides to bust into the room, threaten the serial killer, and initiate combat.

Well then a whole string of nonsense followed that involved one of the other players bear tackling Moore and the murderer while they were struggling to disarm one another and West, realizing he has no combat skills, slinks back outside and waits until the other guys take care of it.

Eventually the murderer goes down and we move on with the story. So fast forward a bit and West has managed to make some impressive checks and while facing the BBG declares "I'm like the main character of this thing!" Which is when the final straw hits me. I proceed to unleash a gigantic abomination that promptly backhands him into the wall, knocking him cold for the rest of the session while I allow Moore to drive a tractor with a plow into the abomination as the final blow.

West was not a nice player and didn't seem to take the hint in the next (and last) session our group had so I had a demon man violate his every hole with spectral tendrils. I am vengeful GM, apparently.