-Friedrich Nietzsche
Most groups who play together regularly eventually consider an "evil campaign", as a way to change the pace of their games and play some antagonists. After so much time saving princesses and defeating dragons, they want to steal, murder, and pillage the town. They want to defeat the heroes for once instead of the villains. They want to play a scheming lich, a brutal orcish warlord, or an insidious drow sneak-thief. On the face of things it sounds like fun, but there are a number of reasons I recommend against it.
ADVENTURING WITH MURDEROUS CRETINS IS A BAD TIME
Evil for evil's sake is neither fun nor realistic, just kind of gross. If the players created their characters so that they could kill puppies, the game will leave everybody uncomfortable. Watching movies that focus on truly evil characters instills emotions in the viewer of disgust, revulsion, and fear. Horror movies focus on evil as a way to alienate the viewer for artistic purposes, but you can't do that in a game. When you, the player, are the villain, this just isn't possible. It's a different experience to cringe watching SAW than it would be to play the movie's villain, Jigsaw.
But maybe the adventurers don't want to do "evil" things. Maybe they just want to be the "bad guys" for once. "Bad Guys" don't really have a reason to stay together as a party. There is a reason that there aren't any major books that follow a group of truly evil characters; these characters don't let you identify with them, because they aren't human. Nobody wants to be in an adventuring party with Sauron (LOTR) because he's one-dimensional and not a fun guy. As the embodiment of evil in middle earth, he has no respect for human life and would do anything for the sake of his own power.
You may be able to get some humor out of an over-the-top evil game, emphasizing the ridiculous lengths your characters go to in pursuit of their misdeeds, but this is tough and very specific. The PC game Overlord did this rather well, but it has niche appeal and doesn't work as well in groups.
HOW CAN I KEEP MY GAME FROM DEVOLVING INTO EVIL CHAOS?
All this said, evil campaigns CAN be run well, but it has to be done carefully. Rather than evil for evil's sake, the characters could be working as pawns to a more powerful entity. They might not be truly evil themselves, but might have to work for the demon who owns their souls, or for the sorcerer who is holding their families hostage. This allows you to include interesting moral dilemmas and give the players diverse challenges.
Alternately, the characters could be working together as a result of rigid power structures. I played an excellent drow one-shot where the party was entirely composed of dark elves, and each character had a "hidden motive" that the other players weren't aware of. Mine was to kill one of the other PCs, a priestess who was working for a rival noble house. It didn't end well for my character, whose botched assassination attempt led to the rest of the party sacrificing him for the main quest, but it was fun nonetheless.
Maybe your party is performing a heist, where their evil actions are strictly profit-seeking. These characters are often present in Shadowrun games, where the characters are all performing
Similarly they could be doing something evil for a greater good. Lex Luthor genuinely believes that the world would be a better place without Superman in it, and orchestrates repeated plots to destroy the man of steel. Luthor is also a huge philanthropist, donating millions of dollars to charitable organizations. Moral ambiguity can be fun to play with.
You can make the game work by having characters with strict limits on what they will and will not do. An assassin might say that he "never kills kids" for instance. Real people have limits, self imposed or otherwise, that form the rules they live by. If your party thinks of themselves as "honorable villains" the game will likely run smoothly. These redeeming qualities can make an evil campaign bearable and prevent it from devolving into mindless, ugly slaughter.
KEEP IN MIND...
Running an evil campaign is different from running evil characters in a "good" campaign. They party might involve evil characters in a party doing good things, or doing morally ambiguous things. If the players are defeating monsters and helping people, they are playing a different game than the one mentioned here. They might be driven by greed, respect for the party's good characters, a debt of honor, blackmail, or any number of other things. The excellent web comic Order of the Stick uses a curse to keep the party's (evil?) character from killing anyone within the limits of an established town. The party might still be entirely composed of evil characters forced to adventure or work for a lawful organization, as well.
Running an evil campaign is also not the same as running a game with a dark setting. World of Darkness games in particular take place in a very dark world, and often involve characters who do questionable things. They aren't necessarily evil though, which is what makes them compelling. A character with evil motives might still be a basically good person. A character who finds their own vampirism repellent isn't truly evil, just deeply flawed. Flawed characters can be a lot of fun to run, but as the DM, talk to your players and nail down exactly what they are looking to get out of the session.
I often find that players who want to play "evil" characters really just want a world where they can do whatever they want without repercussions. While Dungeons and Dragons is about having fun with friends through story-storytelling and escapism, it isn't healthy to take that overly far beyond the bounds of morality. There is no wrong way to play D&D, but take care that the game stays fun and safe for everybody involved. If the players just want to kick puppies, maybe they should take a break from gaming and look at what's going on in their lives.
Most groups who play together regularly eventually consider an "evil campaign", as a way to change the pace of their games and play some antagonists. After so much time saving princesses and defeating dragons, they want to steal, murder, and pillage the town. They want to defeat the heroes for once instead of the villains. They want to play a scheming lich, a brutal orcish warlord, or an insidious drow sneak-thief. On the face of things it sounds like fun, but there are a number of reasons I recommend against it.
ADVENTURING WITH MURDEROUS CRETINS IS A BAD TIME
Evil for evil's sake is neither fun nor realistic, just kind of gross. If the players created their characters so that they could kill puppies, the game will leave everybody uncomfortable. Watching movies that focus on truly evil characters instills emotions in the viewer of disgust, revulsion, and fear. Horror movies focus on evil as a way to alienate the viewer for artistic purposes, but you can't do that in a game. When you, the player, are the villain, this just isn't possible. It's a different experience to cringe watching SAW than it would be to play the movie's villain, Jigsaw.
But maybe the adventurers don't want to do "evil" things. Maybe they just want to be the "bad guys" for once. "Bad Guys" don't really have a reason to stay together as a party. There is a reason that there aren't any major books that follow a group of truly evil characters; these characters don't let you identify with them, because they aren't human. Nobody wants to be in an adventuring party with Sauron (LOTR) because he's one-dimensional and not a fun guy. As the embodiment of evil in middle earth, he has no respect for human life and would do anything for the sake of his own power.
You may be able to get some humor out of an over-the-top evil game, emphasizing the ridiculous lengths your characters go to in pursuit of their misdeeds, but this is tough and very specific. The PC game Overlord did this rather well, but it has niche appeal and doesn't work as well in groups.
HOW CAN I KEEP MY GAME FROM DEVOLVING INTO EVIL CHAOS?
All this said, evil campaigns CAN be run well, but it has to be done carefully. Rather than evil for evil's sake, the characters could be working as pawns to a more powerful entity. They might not be truly evil themselves, but might have to work for the demon who owns their souls, or for the sorcerer who is holding their families hostage. This allows you to include interesting moral dilemmas and give the players diverse challenges.
Alternately, the characters could be working together as a result of rigid power structures. I played an excellent drow one-shot where the party was entirely composed of dark elves, and each character had a "hidden motive" that the other players weren't aware of. Mine was to kill one of the other PCs, a priestess who was working for a rival noble house. It didn't end well for my character, whose botched assassination attempt led to the rest of the party sacrificing him for the main quest, but it was fun nonetheless.
Maybe your party is performing a heist, where their evil actions are strictly profit-seeking. These characters are often present in Shadowrun games, where the characters are all performing
Similarly they could be doing something evil for a greater good. Lex Luthor genuinely believes that the world would be a better place without Superman in it, and orchestrates repeated plots to destroy the man of steel. Luthor is also a huge philanthropist, donating millions of dollars to charitable organizations. Moral ambiguity can be fun to play with.
You can make the game work by having characters with strict limits on what they will and will not do. An assassin might say that he "never kills kids" for instance. Real people have limits, self imposed or otherwise, that form the rules they live by. If your party thinks of themselves as "honorable villains" the game will likely run smoothly. These redeeming qualities can make an evil campaign bearable and prevent it from devolving into mindless, ugly slaughter.
KEEP IN MIND...
Running an evil campaign is different from running evil characters in a "good" campaign. They party might involve evil characters in a party doing good things, or doing morally ambiguous things. If the players are defeating monsters and helping people, they are playing a different game than the one mentioned here. They might be driven by greed, respect for the party's good characters, a debt of honor, blackmail, or any number of other things. The excellent web comic Order of the Stick uses a curse to keep the party's (evil?) character from killing anyone within the limits of an established town. The party might still be entirely composed of evil characters forced to adventure or work for a lawful organization, as well.
Running an evil campaign is also not the same as running a game with a dark setting. World of Darkness games in particular take place in a very dark world, and often involve characters who do questionable things. They aren't necessarily evil though, which is what makes them compelling. A character with evil motives might still be a basically good person. A character who finds their own vampirism repellent isn't truly evil, just deeply flawed. Flawed characters can be a lot of fun to run, but as the DM, talk to your players and nail down exactly what they are looking to get out of the session.
I often find that players who want to play "evil" characters really just want a world where they can do whatever they want without repercussions. While Dungeons and Dragons is about having fun with friends through story-storytelling and escapism, it isn't healthy to take that overly far beyond the bounds of morality. There is no wrong way to play D&D, but take care that the game stays fun and safe for everybody involved. If the players just want to kick puppies, maybe they should take a break from gaming and look at what's going on in their lives.