I'm guessing the emperor is actually that large fire dragon we see at the start. Maybe he reincarnated with a similar wish for revenge but has found himself stuck inside an immortal human body which causes him to suffer daily. That would explain why he doesn't mind killing people so easily and why he killed the Knight as well. He probably even thought he was doing MC a favor.
Rather than wife grooming, I'm guessing he's actually assassin grooming MC. The empress with her psychic powers foresaw her being the white dragon, the most powerful of all dragons, and thus the only one capable of defeating.
It confuses me that this series is actually called the careful empress when MC is actually the queen. I wonder if the empress has come from another world and, rather than being a seer, is actually just aware of how the story goes. That or maybe she's going to end up being the main villain all along...