In multiplayer games like MUDs and MMORPGs where there is an affiliation that's clearly the "bad guys", is there any trend in who tends to join it?
So before I launch into anything potentially inflammatory let's start with the disclaimers. Anything one says about a general trend about a large group of people cannot be applied to any individual member, particularly as regards motives. There's gobs of room for an individual to deviate from a loosely-correlated trend. There are lots of reasons why one might choose one side over another, including interesting skills or tactical opportunities that might exist in one side, having a friend in one side, and many others, which will on an individual basis easily overwhelm any small predilection. Those who play multiple characters are particularly prone to trying every side out. Some games use "good guy" and "bad guy" as the thinnest of veneers and don't really bring along with them very much baggage about how one behaves or what motivates the characters, so this hardly matters there. And in a short-term or one-shot game the other factors, including a desire for variety or a new challenge, easily outweigh any predilection.
So with that out of the way... from my experience, those who play the bad guys are substantially more likely to be jerks. By "jerks" I mean the kind of players who are more likely to selfishly pursue their own enjoyment at the expense of other players, instead of finding ways to help contribute to the benefit of the whole game and all its players. They're more likely to rationalize bad behavior or go too far.
Sure, in any given "bad guy" organization there are plenty of people who care. And in any given "good guy" organization there are plenty of people who are jerks. And in either one, there are people who are one way but claim or pretend to be the other (and may well believe it themselves). But there does seem to be a small but significant skew, where more of the jerks find their way to the side where being jerkish is an appropriate thing to do in character.
This is hardly that surprising, since in any given group of roleplayers, most of them will be mostly playing themselves in fancy clothes and with superpowers. This is particularly noticeable when it comes to characters you're going to play for a long while (years, in a MUD, for instance). I can easily enjoy a character that is unlike myself in key ways for a long time, but when it comes to playing characters who are mean-spirited, evil, cruel, or hurtful, I can only enjoy that in short doses. (I once played a wife-beating escapee from a prison for the criminally insane for a few hours at a con, and enjoyed the challenge, but I wouldn't want to do that for even a few sessions.) But it seems that some people can, and I think that might tell us something about what they're like inside, or what they would be like if they thought they could get away with it.
This is sure to be an unpopular opinion as it comes off as judgmental, and I am sure that the desire to proclaim the exceptions loudly will tend to outweigh and drown out the correlation of the rule. But I think that, for those who know what "slight but significant correlation" really means, it's a potentially significant fact.