Maybe it means you choose to be the bad guy, even though everyone in the world is a bad guy.
One of my complaints with Fallout 3 (still not finished, or even deep into the main plot) is that there aren't nearly as many options when trying to play as a bad guy. You can rarely bully people into doing what you want, and killing people just makes everyone around you instantly hostile. Plus if you help someone, they give you quests. If you kill them, you miss out. Maybe I haven't been around enough but I'd like to see quests become available based on my karma, so that being bad doesn't feel like it's locking me out of stuff. I feel like the game is forcing me to be chaotic good, when I want to play chaotic evil. (I'm not sure if it's nerdier that I used those terms, or that I didn't know them, but looked them up.) So I feel like I'm forced to follow the rules until no one is looking. People thank me for my help and smile when they greet me, then I steal shit behind their backs just to artificially lower my karma.
In FO3, I feel like I'm going to miss out or screw up crucial plot elements if I play full on evil. Example from early FO3 (Moriarty early plot stuff): [spoiler]Moriarty is an asshole, and I wanted to kill him. First time I talk to him, I did. The whole bar went hostile, I escaped, and the whole town was hostile. I almost made it out of town, but then what? I wouldn't know to head toward GNR, because Moriarty didn't tell me. I'd be lost, because I chose to play the evil guy who shoots the asshole in the face.[/spoiler]