
That Would be interesting, but what if it's a real medical condition? There is a medical condition where a person can't feel pain. They're nerves have been disconnected from it. If the emperor has it then it could have caused an empathy disorder because for people to have any empathy they have to be able to picture themselves in the same situation, and if the emperor can't feel pain then he can't imagine what others are going through because of his actions.

Yes, hope they will explain the emperor's personality soon. A tough childhood, an illness, a trauma? Whichever the reason is should be shown. Hopefully it's enough to be a reason. Also, the uke seems to be forgetting to hate his brother. Why is his hatred fully given to the emperor? Hopefully that would be tackled as well. I heard the novel has a happy ending so I assume this manhwa would have one as well.

Authors usually give a good reason or just kind of smooth over it unfortunately. I'm more of the Luke's handling of the change because so often I read that the behavior is just forgiven and swept under the rug but not properly taken care of.
I think the brother isn't having much of a spot light at the moment because what he did was bad, but it wasn't as heavy or long-lasting as what the emperor did. The brother had two instances but then the like ran while the emperor had multiple instances over YEARS so I think the brother was just overshadowed by the emperor.
So the emperor has a certain illness? Incapable of feeling pain? Emotionally detached? He mentioned about northern tribes or something. Perhaps it's an illness or a curse, and only by loving someone would he get to experience pain and suffering. That's why it's easy for him to slaughter people and blame it all for the sake of love. Then maybe our uke has a complex origin as well? Since his brother said he attracts men with his body and he couldn't resist touching (and raping him). That's awful if that is the case... Well let's see how it goes. I hope this story will be long enough to fix everything and give us a good reasonable plot until the end.