This is the downside to pug dungeons/events/lfr's, whatever. Yeah, sometimes you get jerks with no concept of reasonable expectations, and just as often you get jerks that just show up to things, auto shoot/attack, or flat out afk them as well. It's the price of playing with strangers. We've all had to deal with obnoxious players with giant egos, or (even worse in my opinion) dead weight players who just expect someone else to carry their load so they can get free loot. MOST reasonable players in a 5 man can tell the difference between a useless lump of garbage who wants to be carried, and a new 90 who is making an honest effort. I've never once yelled at someone who's trying, but until Blizzard or someone in the addon community figures out a way to measure effort, mistakes will probably be made, and undeserving people will get yelled at by irrational a-holes, and good players will be forced to carry useless parasites looking for a free ride.