What kind of evil?
Power fantasy of abuse and subjugation?
Have them play vampires or demons. Awaken from their slumber in a small town and go about setting up a cult and securing their safety. Make thralls out of people. Add some sort of mechanic like "eating someone's soul gives you a stat boost" so mechanically they're rewarded for cruelty. Have NPCs beg for their life. Have some sell out their neighbors and loved ones for favor. Let the players kill them anyway.
Maybe some heroes show up to fight them and free the people. Maybe it's just two hours of crushing limited resistance and making their temple.