Ummm

didn't you see the part where he got reminded of the past lover by the mc pretty typical ml type shit besides we don't even know what happened in the past to make him harden his heart like this so I wouldn't judge to fast

If the girl that died isn't the og ranhee seo, then def ranhee seo and the dethroned empress got something to do with it. Maybe the emperor is lowkey punishing Ranhee Seo by ensuring that she doesn't experience happiness, and by that by forever locking her up in the palace full of trickery and betrayal. He mentioned before that mc can be his and his alone, meaning that he doesn't want to let her go, either Ramhee Seo dies or he dies before she can experience freedom. He sorta wanted the mc to experience the same loneliness, together experiencing that same life long loneliness of not having someone they love. The emperor, being the one who lost his loved one in the past, and Ranhee Seo, being unable to obtain and experience the emperor's love, is in itself torture for both of them.

If this is really the plot, this is awfully a bittersweet journey for both of them.

But I guess that lady who said I despise you to the emperor died it can't be ranhee

I thought that maybe the emperor is not another player, but the game master who reincarnated from his past life. But seeing how both his mother and that blie haired consort know his wounds, I guess it IS his past and it IS NOT our mc, Ranhee Seo