Evil. In some worlds it is something that can be carried around in a bucket. In others it’s more ephemeral; abstract. In others it is simply the glue that holds the setting together. What’s a GM to do?
Largely this depends on what you and your Players want out of the game. I mean evil exists differently in different systems so you have choices on this subject at every stage of the game. Do you like ambiguity in your games or do you prefer a more black and white approach? Ambiguity tends to make for more role-playing as there are more decisions to be made but, as many an apologist will tell you, the shortcomings of any system can be remedied by a good GM. You always have the option of taking a black and white system and smudging the edges.
What do I mean by saying evil exists differently in different systems? In White Wolf’s World of Darkness (WoD), there are no alignments per se and everything is point of view. So everyone is evil from someone else’s point of view because, much like in reality, every group is sort of out for themselves. In WoD, as in reality, there exist things like the question of nature vs nurture, free will, remorse, and redemption. Someone can be a bad person one day and change their lives the next or vice versa. In D&D they have alignments and even places or inanimate objects can be evil by nature. (See that rock? Yeah that one. It’s evil. Really? It’s not harming anyone over there. Oh, but it wants to…) In a setting like that creatures can be born evil. That’s right. You can have a whole orphanage of evil infants that a Paladin wouldn’t bat an eye at burning to the ground.
So the first question is what kind of role do you want evil to play in the game? Do you want it to be a solid tangible thing or do you prefer a more realistic approach? The next question is character generation and this question is directly affected by the previous one. If you chose a black and white world, then you’ll have an easier time setting your PCs up to be on one side or the other. In an ambiguous world the most you can really do if you desire to avoid an evil game is to have your Players explain their PCs to you before the game starts and make a judgment call on whether or not they’re “too evil.” PC1 remorselessly takes life force from people to fuel their powers. Never enough to kill, but doing this noticeably and permanently lowers the quality of the victim’s life. PC2 murders regularly, but because of their nature they must murder to stay alive and they display honest remorse each time it happens. PC3 finds killing to be distasteful, but that’s because they are a trained, expert torturer and having a client die on you is simply unprofessional. Which of these three PCs is the “evil” one? Which one is “too evil”?
Think about the tone of your game. Do you prefer actual heroes beating back the forces of darkness and taking the high road in all situations? Do you prefer antiheroes who also beat back the forces of darkness, but believe the ends justify the means? Or do you not see the point of beating back the forces of darkness at all and prefer a game of self-interest or even a game where the protagonists are fully evil?
Personally I’m not fond of games where evil is acceptable. I like my PCs to at least think they’re doing the right thing from their point of view. Antiheroes are fine but for the most part I try and influence my Players to make characters who at least have non-evil goals. Selfish is workable, but no objectivists (as in life, too much selfishness is just uninteresting). To accomplish this I have to think carefully about the merits of being good. The merits of being evil are obvious (Take what you want. Give nothing back. Yo-ho-ho and a bottle o’ rum.) The merits of good are harder to see though because they are larger and more long term.
The most important benefit of being good is trust. Two good people can trust each other and work together through thick and thin. With two evil people, eventually one of them will turn on the other because no two people’s goals always align 100% of the time. They’ll get by for a while based on the idea that being together is safer than being on their own, but all it takes is one of them to get an offer for the same arrangement by someone who is better off (and possibly more trustworthy) than their current partner and then the bond crumbles. In this sense good is evolutionarily advantageous because it allows for large, loosely controlled (and therefore creative) societies. To keep an evil society together, there must be some strong system of control that would by its nature reduce the happiness of the populous and hinder free thinking creativity.
Now how do I translate this down to the level of individuals to make such a thing tangible on a PC’s level? There has to be a noticeable difference in the interactions with evil and good NPCs. Evil NPCs burn you. Good ones don’t. Good ones can become your friend and eventually desire to help you for that reason alone instead of only for compensation. Evil ones can not as they respect only power and compensation. The instant your pockets are empty or your power in question, they abandon you. Also, Good NPCs will only give their bonus benefits to good PCs. They know that evil will burn them and so don’t bond with those types.
In other words, I let evil PCs workout for themselves where the benefit from being evil comes from as that’s part of their chosen package. Good PCs I allow the option of being given bonuses by me because cooperation is part of this particular package. Besides, all evil games are boring. They start out novel and weird as the PCs just ransack their way through life, but eventually all the scams have been run and all the patsy have been stabbed. Because the nature of evil is to think short-term, if evil is the rule in your game eventually every session becomes a wash-rinse-repeat affair and that’s quite lame, in my humble opinion.