Responses

I don't think he did it out of fun. I think it's possible the emperor also had some kind of deep resentment against his concubines/consorts carried over from previous life. There were a lot of court intrigues happening yet to be revealed, and Heun used to be targeted a lot by these women too. So I think that hatred just subconsciously manifested in him when he sleeps because he didn't remember yet. As heartless as this emperor is, he is not yet at Sangwoo's level of craziness.
even if he does change dont forget hes a psychopath who used to kill the person he slept w out of fun