I don't think their should be a limit to how evil you can be in a game. If you are playing as a villain protagonist then he or she must be shown to actually be evil, not have it as an informed ability.
Rape, murder, torture, genocide. It's all very well telling the player that their character is the lowest scumbag on the planet with no morals or human decency of any kind, but if you don't show it how exactly he or she would beat a child to death, or eviscerate a puppy, I can't really appreciate it. Having me complicit in evil acts will drive home just how thoroughly evil the PC is supposed to be.
“Darkness cannot drive out darkness: only light can do that. Hate cannot drive out hate: only love can do that.” - Martin Luther King Jr.