tinyE: Carmageddon doesn't bother me for some reason though, or Postal. Hmmmm, better tell my shrink that one.
I assume these games don't do anything to make you get too attached to the people in them, they don't treat them as many-facetted personalities but rather deal in stereotypes or disposable dummies, and maybe even reward you for "evil" or chaotic deeds. Most story-telling RPGs punish you for being evil, even when you're allowed to play a villain and even when you're rewarded with xp and items for evil deeds, because at heart they're still moralistic and want you to believe that their worlds are full of "real" and "deep" characters with feelings. You might argue that this isn't the case with your Diablo henchman, but maybe you get kind of attached to him because he's helping you and the advantage in protecting him is greater than letting him die (which you might register as failure on your part, not as something amusing, since it's more challenging to keep him alive).
Anyway, that's what I meant when I was talking about game design. Most attempts at letting you play a villain are half-hearted because the devs are not fiendish enough to really mean it and try to make players feel good about the bad things they're doing. And the more a game relies on story-telling, the harder it becomes to create plausible ways to seduce the players to act destructive towards the rich world the story-telling creates, and most of the times, the authors don't really want you to destroy it (while at the same time thinking that destruction is the only possible face of "evil", everything else would be too complex).
ChrisSD: Oh and even "good" characters seem to have no problem taking anything that isn't bolted down ;)
Yeah, that's a good example of how games can train players to act good or evil and feel good or bad about it. Stealing and looting isn't registered as evil in RPG, as long as noone reacts to it, so players can both feel good and do bad things - because these things don't have any real impact in the game and players are not confronted with the consequences of their actions.