In a few Video Games / MMOs I've seen, players have voiced interest in being the bad guys of the setting, but I also recently read that in some games where this is allowed, players generally find they don't actually enjoy it for very long. The usual reason I've been given is because if the character is truly evil, and not just a 'faux evil' sort of thing, the players find they have a hell of a time really having sympathy and interest in the character they're playing. I think, perhaps, what these players want is the 'freedom' that comes from playing characters who don't have to be nice and good -- but don't want to actually jump fully into the 'evil' camp -- they want to restrain their evil actions to something they can actually stomach.
This got me thinking about characters in roleplaying games, and what kind of adjustments a game master might have to do when dealing with evil PCs. The first thing I'm considering is Alignment (in D&D settings), and wondering just how many people are willing to step fully into Evil (rather than just Neutral). Basically, how evil will most players be willing to go, and how comfortable would they be doing so? In my experience, I've actually tried Lawful Evil and Neutral Evil -- the tyrant who is willing to do horrible things to ensure order, and the selfish bastard who is willing to use anyone to get what he wants. They had their limits of course, but for the most part I would notice they would flutter closer to Lawful Neutral and True Neutral with the odd act of 'okay, that was EVIL'.
The second thing I think about is the social setting. If you're playing it straight, most people aren't going to be comfortable around the characters. How much does the game master allow the reputations of the PCs to affect people associating with them? I don't think most players will enjoy 'yeah, nearly everyone is avoiding you' as a status quo -- and of course, if they use threats and force to get what they want (even something like buying goods at a local shop), it would logically make things worse for them, rather than better. The consequences of being evil should be there, but how much should the GM hold back for the sake of the players? Personally, I'm rather inclined towards 'you get the consequences appropriate to your actions' -- which I think is also why most of my players would be willing to make reasonably selfish characters, but never characters which would truly be considered 'evil' in my view. I've only recently ever had one player make a character I would consider Evil -- and the other characters could barely tolerate the character -- let alone the NPCs.
The other thing that I'm considering is more towards fantasy settings. If the PCs gain a bad enough reputation, there would be NPC resistance. I'm thinking of things like city guards, or the region's knightly orders, or paladins, or NPC 'heroic parties'. This becomes a balancing act -- most people are familiar with how heroes and Good characters deal with evil threats, but what happens when the PCs are the threat? Do you play it straight, or do you pull things back for the PCs? I'm more inclined to playing it straight -- look at the tactics heroic PCs use to hunt down the evil that lurks within the city or region, and just flip the mirror. If the PCs don't want to be hunted like dogs in the streets by paladins and heroic NPCs, they need to be careful with how their reputation spreads.
On online games I've dealt with, this has been a constant complaint -- evil PCs getting mobbed by good PCs on the same server. The usual complaint is 'this place isn't Evil friendly!', and my usual response was 'of course not, most people want to be the hero'. The other thing I'd answer is 'you have to be a higher grade of evil'. In these kind of settings, the evil characters who get the furthest are the subtle ones -- the ones who hide their evil behind a veneer of civility, or who aren't able to be caught doing anything actually illegal (though the outcome of that was one of the PCs breaking the law specifically to blast the person out in the open anyway -- a case of "screw the rules, I know he's guilty, murdering him is the only option"). I've seen some players complain about evil PCs who wouldn't leave any evidence of their wrongdoing, but in a world that's geared towards heroes, that's pretty much what you have to do if you want to survive. Some players may like that cat-and-mouse game, but what about those players who don't? Who want to be the 'larger than life' bad guys?
Something I'm going to have to think about more.