Running an Eeeeeevil Campaign.
Sooner or later, you get a group that is deeply committed to doing this. From my experience, it’s not as easy as it seems; and I’ve been on both sides of the screen, trying to do it.

The first problem is that players invariably decide that Evil = License to do whatever they want with no consequences. This has to be nipped in the bud, if you want a campaign that lasts longer than a couple of nights. Few systems are actually set up to make PvP very functional, and few groups can handle the internal intrigue of plotting against one another without imploding. Regardless, communication and character separation is important, perhaps more so in this kind of game than in any other.

The second is keeping a group of evil characters together and motivated. Now, I have seen this done; the easiest way is probably to have it be Divine Ordinance, with some kind of geas enforcing good manners. Whatever your method, the consequences for breaking faith need to be immediate and harsh; the group has a job to do, and they will do it, or their heads will explode. Possibly quite literally, ala Suicide Squad.

After that, all you need to do is remember that Evil isn’t necessarily the mustache-twirling villain trope, and that the players do need to have some flexibility in how they accomplish the thing, as long as they’re not preying on each other. Evil is also not necessarily stupid; good wins the day in most shows and movies because it’s scripted that way; as real life will prove, things don’t always go like that in real life.

 

When you are writing a villain, whether they’re a nastier opponent for your group of ne’er-do-wells, your actual ne’er-do-wells or a regular antagonist for a nicer party, it’s good to remember that said villain will have drives, needs and issues, just like the good guys. Your villain or villains shouldn’t be doing things just because they can. Even dragons don’t kill solely because it’s fun; they slaughter villages for revenge, eat knights in shining armor because they are made of meat and run small countries because it’s easier to get people to give them taxes and tribute than fighting said knights and their inevitably chewy backup for whatever trinkets they might be carrying.
Cultists don’t always want to destroy the world; sometimes they’re greedy bastards who want to rule it, instead. Again, getting people to willingly hand over a regular share of their wealth is much easier than killing them and taking it.
All they need to do is get to that point and that? That’s another story.