I'm going to have to respectfully disagree with you, Ronin. I don't think the transition was too abrupt. You must take the entire movie into mind as well. He starts off by answering stupid questions, an obvious source of frustration for him. When the troll appears and makes him realize he's just wasting his time, it solidifies his negative view on the idiots who have asked him questions previously, but also gives him hope and purpose as he heads out to find those with the intelligence to benefit from his wisdom. However, the extremely negative, and often times childish, response to his attempts to help gradually cause this hope to erode. Finally, while in the dungeon, it's made clear that they've been standing at the entrance for an unnecessarily long time. (Tank had to go get the right gear set. Mage went AFK. etc.) While this event on its own may not have been enough to cause him to snap, when piled on top of his dwindling opinion of others and his growing frustration and impatience, it becomes the proverbial straw that broke the camel's back. Thus, rather than attempting to give advice that he knows will just be ignored or rejected (Such as "You should be sure to grab the right armor set BEFORE queuing up,") he snaps and expresses his frustration and impatience in a simpler manner.

Go Go Go Go Go Go Go Go!