Games often make it blatantly clear whether you're playing the good guy or the bad guy; if you're the good guy, you run around fighting bad guys. If you're evil, you run around fighting good guys.
It just doesn't work this way.
Very few games actually manage to convince me that I'm actually being an evil person, and rather convince me that I'm playing a good character with reskinned enemies and a different story. This is exactly where games go wrong; by actually telling the player what to do, they completely violate the core rule of evil; it doesn't play by the rules.
Let me simplify it for you; being evil should never be a choice presented by the game, it should be a decision made on the players own time. There should never be a physical option in a game of choosing between a good campaign and an evil campaign; evil is undefined. It has to be decided by the player.
Oblivion utilizes this concept quite well; you are never asked whether you'd like to play an evil character or not, it is simply something you decide to do based on game mechanics. You have the option of killing anyone and taking anything, and what you do with that is up to you. Additionally, upon doing so, you receive the consequences that come with it. That is the second important idea; evil only exists by breaking rules, which only exist because of the consequences that enforce them.
Take Bioshock as an example of what not to do. In Bioshock, you can either kill or save the little sisters, choosing to be good or evil. Are there any consequences other than a little cutscene for being evil? No. And by presenting this as an option with no consequences, the game has basically told you that you are following the rules, and thus, no sense of playing an evil character is felt.
Does this mean games should never suggest being evil, then?
Of course not. However, they can still make you feel evil while keeping it as a main part of the game. Take GTA. You can kill people and steal cars, and doing so is a large part of the game. However, you could also not, and not have the cops on your tail all the time like they would be if you had killed someone or stolen a car.
In conclusion, stop asking me what side I want to fight on, and just hand me the damn gun.