One of my players wants to betray the party and join strahd

He said the he sick of them always messing up with situations like talking shit about strahd in front of his minions and other situations like that He said he wants to betray them in the last session when they get emulate and the sun sword and wants to give them to strahd what should i do

Edit: I made Strahd a Chill guy a like a character from the office but isn’t to be taken lightly He tells dad jokes from time to time but if you annoy him isn’t hesitant to punish you I made him kill on of pc for being annoying and then reviving him when the conversation ended all the players enjoyed that moment even the dead one Made him cook a pcs(the player this post is about) mom and serve it to him in dinner Made one of his brides a pcs long dead wife.