Assuming it's the same GUEST all the way through in the threads Jim linked us to, I wouldn't call that a troll. That is because it seems pretty clear it is someone with a real interest in the music, and in tye point at issue. A prickly and ill-tempered someone, and there's probably a word for that as well. Pillock is one that comes to mind.

(And the paradox is that on the actual disagreement that blew up, I'd incline to have some sympathy with the nameless GUEST's position - none whatsoever with the way it was expressed and developed.)

The nameless aspect here turns what would normally be a spat between two people with a different point of view into something worse. It makes it impossible to shake and make up, even if both parties wish to, and I'm sure it encourages people to be nastier than they otherwise might.