The Thief, D&D's biggest jerk class.
I think I have finally figured out why though. The name of the class says "If you are a player that wants to screw over the rest of the party, play a Thief!". I know Conan was a Thief for a good portion of his career, but a lot of D&D players have never read Conan, or Fafhrd and The Gray Mouser*, or even Thieves World with Shadowspawn, so, even though AD&D doesn't come out and say "Steal from your party", it does mention that most Thieves tend to be Evil and they see things like the drawing of the shirtless Thief robbing somebody at knife point in the 1st edition Players Handbook right under Thieves XP table, or the "There is no honor among Thieves" drawing in the alignment description section of the Dungeon Masters Guide and they assume that's the code of conduct for Thief characters. The Moldvay Basic book comes right out and says it "..as their name indicates, however, they do steal- sometimes from members of their own party". Dr. Holmes states a little more bluntly in their class description "Thieves are never truly Good and are usually referred to as Neutral or Evil, so that other members of an expedition should never completely trust them and they are quite as likely to steal from their own party as from the Dungeon Master's Monsters.". When you couple this with the fact that they have that Pick Pockets skill, what are they going to do? Screw the party, that's what they're going to do. They don't have to, they level faster than every other party member, so stealing the gems or some extra gold so the get bonus XP they don't have to share is just damned greedy, but they do it because they can, the system encourages them to.
Kind of a dick, eh?
So I got to thinking about this, if D&D is fantasy F-ing Vietnam, then Thieves are like the Tunnel Rats, or SF or some kind of specialized warrior minus the name-tag and the Pick Pockets ability; because when you think about it, what else have they got? Find and Remove Traps? That's a pretty awesome and helpful ability to have in a party, very Tunnel Rat-esque too. Move Silently? Also damned handy and kind of militarily helpful, in a stealthy commando style. Hide in Shadows, same thing. Hear Noise, again, same thing. Climb Walls, again, same thing. Open Locks is the only iffy one there, and I can see an argument for it being a militarily useful skill, or at worst, an espionage type skill**. Even their Backstab ability is a pretty bad-ass commando type ability, so these guys could have been called something else (like, say- "Sneak Attack") and saved us all years of intra-party conflict and douchebaggery.
I guess not every D&D Thief needed to be played like a raging douche, they could have been played as though they were modeled on Indiana Jones, he displays pretty much every single Thief skill (including Read Languages that I didn't mention because I was reading them out of the Moldvay Basic book and the Holmes Basic book when I listed them), but sadly, no, he's an Archaeologist, not a Thief (although that's a subtle distinction depending on when and where you are and who you ask); all I know is I'd rather have Indy in my party or a Tunnel Rat from 'Nam than any damned Thief, even if it is Conan. Conan occasionally screwed over his party***.
*I haven't for instance.
**Which brings me to the sad slippery slope argument that Pick Pockets makes for a great espionage type skill too. Maybe it would work if the Class wasn't called Thief and didn't have all of those references towards stealing from your own party and tending toward Evil.
***To be fair, if memory serves, they were usually Thieves that were planning on screwing him over, but you have to be careful about the company you keep.