You could have the PCs be in service to an evil wizard or overlord, and just take any regular plot and give them the baddies' role.

You could have multiple factions in a city, like noble houses, and give them some sort of goal to accomplish in that context. Think Game of Thrones or the drow in Menzoberranzan.

You could have them doing something "good" but with a "the ends justify the means" mentality.

pathfinder's begginers box. sorry. just being a fanboy ass.

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.

a town is planning a yuletide or similar celebration where they all pooled their resources to get an expensive icon. have them stake out the delivery and go into the town to steal the item, murdering anyone who resists

Ask your player what they mean by evil in this context. Do they mean evil as in oil barons extracting all the wealth they can by any means necessary, or does she want to skin babies to make purses?

How about a "Suicide Squad" like run. The players can create all kinds of evil prisoners that get send to kill a thing that all the heros already failed to defeat. Now they can try do to what they are told to in exchange for their freedom or whatever or try to run away and and not risk their own life's on the way.

A lot of chances to play evil or even reconsider when the little orphan begs for help.

A heist job.

The PCs are forced to kill, whether or not they want to.

At a pivotal moment, an NPC from one PC's past shows up - or maybe just somebody who convincingly pretends to be one. They reveal information that makes it sound like the PC has been conspiring against the rest to split the hall with the NPC.

Thing is, the NPC is lying. But is very convincing.

In effect, you orchestrate a situation in which evil characters begin to infight. And because they're evil, it creates lots of opportunity for highjinks.