I've met some ppl like this, and i've notice they're the ones that have no clue on what they're doing. They're just covering it up by flaming someone. If they don't have any constructive criticism for you they're just trying to make themselves feel better.

I would say don't stop doing PUGs because of this person, because you can get to know great ppl with PUGs. If you ever come across another putz just blist or ignore ^^