In Fallout 2, you can help enslave people for money as a slaver. You can kill innocent men, women, children and dogs, many different ways. You can take advantage of desperate people. You can marry and pimp your wife for some extra money. You can become a gangster. You can back stab people. And after doing some of these things, the world will start to see you as evil and people will hire bounty hunters to kill you. You can even set up booby traps for children in this game... Sure, you have to try to save your village in this game so you can't be 100% evil but you can still be 99.9% evil.

Far out, thanks for the info. I've only ever played #1 but #2 sounds like fun. :D

Fallout 2 also has more jokes in it and things that make the game less realistic but more fun, like a car you can drive around in. Your character can also be a porn star in the game and a famous boxer too. The porn star part is hilarious when you have to pick your porn star name from this list the game gives you.

Keeveek! Where've you been, man?

On the contrary I always use drugs when they're available in life.

Fallout 3
It allows you to choose the sex and how you behave towards others.
You can blow up a city, slaughter hundreds for your own gain and enslave people whom you catch unaware, plus there are many other decisions you can make along the way. Although the alignment does not make people behave completely different and the story continues along irregardless of most of your actions, you can still choose how you act in a given situation.

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Fallout New Vegas. Of course you can go all evil incarnate, but no matter what you do, you won't be a true hero. NCR is a friggin' dictatorship, though it's a decent alternative to the chaos of the wastelands, Caesar's Legion is a slave based/survival of the fittest society, Mr. House is a corrupt old man and you controlling an independent New Vegas, not exactly a good ending for everyone.

Not really evil, but unique and dark (and certainly not your every day heroine): "Elvira: Mistress of the Dark" (in the the sequel "Elvira: The Jaws of Cerberus" you get to rescue Elvira).
Both games sadly are not available on GOG.
[url=http://en.wikipedia.org/wiki/Elvira:_Mistress_of_the_Dark]http://en.wikipedia.org/wiki/Elvira:_Mistress_of_the_Dark[/url]
[url=http://en.wikipedia.org/wiki/Elvira_II:_The_Jaws_of_Cerberus]http://en.wikipedia.org/wiki/Elvira_II:_The_Jaws_of_Cerberus[/url]

What about anthropomorphize none genderspecific things, like robots?

If that is the case, I like HK-47 from Star Wars: Knights of the Old Republic, but he isn't your main playing character, but a party member.
You of course can still play your main person as something something something darkside.

Smugglers V, def.

edit: also Chaos Overlords.

Saint's Row 2. You can be male or female, customize your look and clothes, and you're a gang leader.

Thief 2X: Shadows of the Metal Age.

Fanmade expansion/mod for Thief 2 featuring a 13 mission campaign of revenge for a new female lead character.