I'd figure what qualifies someone as a jerk is a situation where:
A) the celeb WILLINGLY placed themselves in the position of being accosted by fans (IE: a con) and was rude to people
B) the celeb was in a private setting, but their degree of irritation at the fan intrusion is WAY over the mark (IE: that story about Ryan Reynolds throwing someone's sharpie on the ground and bitching them out for asking for an autograph)
or
C) when the celeb is crappy and rude to people who are trying to facilitate their job (IE: the stories about Julie Benz being rude to convention workers who are just trying to be helpful)
Situation A there's just no excuse for. You're there at a con, being paid, and making money via autographs and photos, probably. Put on your ****ing party hat and at least ACT happy. Hell, you're an ACTOR, so start ACTING.
Situation B is a bit trickier. I mean, on the one hand, you're just a human being who probably would like their privacy. On the other hand, you also signed up to be a celebrity, so get used to dealing with "the public." If someone politely requests an autograph, there's no need to freak out on them. The amount of energy it just took you to flip out was probably more than just putting on a fake smile, signing the friggin' photo and going on about your business. Now, if you're SO famous that you'll get MOBBED as soon as you sign for one fan, I can understand, but in just about all of these stories, the stars haven't been so big that suddenly they'd be completely surrounded by screaming hordes of fans. Regardless, a simple "Sorry, I really don't have time" or "Not today, sorry" and a smile may be enough. If the person keeps pushing, ok, I can see getting less polite, but at least START polite and go from there instead of just going from zero to jerk in 6 seconds flat.
Situation C there is, again, NO excuse for. If you're crappy to the crew on your show/film, or the people working the con or whathaveyou when they're just doing their jobs (IE: NOT what happened in the Chris Lloyd story), you'd better have a DAMN good reason. Personally, I though Christian Bale had a legit reason to give the grip some grief when he helped kill the mood of an intense scene....albeit not to the degree we heard on the tape. So again, there's a question of degree and the crewperson's conduct. But that said, to treat people like peons...not cool. Not excusable, and it definitely makes you a jerk if you do that.