Why was he even sleeping with this creep in the first place?

Informations... i guess(?)

To get information on jeong mo. Jeong mo is this "creep" uncle. Jeon mo also had affairs with Yeon jin's father sexually and business related.
So, as yeon jin is about to get revenge on his father, he is keeping this creep close.
I hope it would help (￣∇￣")