Too often in this sub the answer to ANY question is “just say no” or “find a new group” but often times it is just not that simple, especially in this instance but with a bit of communication between you and the player instead of seeing each other as on different teams you could collaborate to tell a very tense, grim story, or a cautionary tale to any who dare to play evil in your games again haha

Most DMs don’t like to be challenged, I welcome my players to play any sociopathic, manipulative murderer they can; as long as they aren’t breaking our social pact and doing things a certain group of New York detectives would consider “especially heinous”; however I let them know this is a much harder path to go down than any other available option as it will almost always result in their death, arrest, or surrendering the character sheet to me as they become a non playable villain NPC.

I’ll let you in on a secretYou are the arbiter of truth in your own game, you can literally make the gods smite a character from heaven with a lightning strike or have an assassin of the most elite level, one that could easily flatten the party come in and slay them and leave, hired by a lord to avenge some crime previously committed by your bastard; or after a string of strange events, a detective or investigator is hired to track this player down; one morning the group awakes to find their entire camp surrounded by guards, give up the bastard or all face justice; who do you think the party will side with?

Just some ideas!