Eugh, the assumption some people make is just downright wrong. Evil doesn't imply freedom. It's quite the opposite. You have to live with the thought that you cannot show your true self, lest you be hanged or put into prison. Some characters will show off with their means, that's right. But this doesn't mean it's normal in the spectrum of role-playing evil characters. It means players behind these characters suck at role-playing :P The good characters have it covered. They can live however they want and the society will accept it; They don't need to hide with anything (unless the environment makes it that there's a tyrant they wish to overthrow).
Basically when it comes to "show off" evil characters, one can be either uber powerful or dead.
The former for the most part was put to absurd levels by this nice invention called "internet".
I have been playing an evil character in Warhammer: FRP system. Not much shock there, as most characters there turn out either chaotic (as in unlawful) or evil (though in 2nd edition there's no more alignments).
While it was nice and all and I managed to develop system of ways to cover my shady actions, eventually the character has grown too strong. Together with his evil inclinations, you would see him wiping out large numbers of enemies, who wouldn't even know who attacks them. Needless to say, the game became boring both for me and other players. So after some time I used new events that came up on our sessions and moved him more from chaotic-evil to chaotic-neutral alignment. Together with that I made him develop fear of using magic (paranoic fear of witch hunters and himself fearing for his newly reclaimed sanity was good for that). In the end he was still a shady character, but prefered killing methods more akin to cutting throats or poisoning.
Moral from the story: Uber-powerful evil characters are downright boring.
I think for me these three alignments are the most interesting to play (using d&d system):
Chaotic-Good (the misunderstood)
Chaotic-Neutral (shady, but not murderer)
Lawful-Evil (law-abiding wrongdoer)
Other alignments don't really require much skill. In context of this thread:
Neutral-Evil (steal and kill if you need to)
Chaotic-Evil (Ragh! Murder!)
I don't see how the above two could be considered "hard to role-play". And while I think I even came across interesting NE character (if one of Undie's is such), I am still yet to see someone pull off an interesting CE character.
Anyway, to answer the question, I think for me the most motivating is the game environment. For example: In WFRP it's very easy to wind up with shady characters, but in Planeshift very often it's kind of weird to play anything of this sort. That is considering how "friendly" the environment is. Cheap weapons, easy earned money, rightfull government. The world just isn't dark enough to play evil character without looking somewhat stupid and out of place (at least for the majority of the "Evil" spectrum). Death Realm is meant to be the place where the "Evil" may thrive, and to be honest I can really see proper mood in that place. But it's probably something that won't be developed to proper size for a very long time.
