I don't think this demon is all bad. I think there is more to the story and probably the jewel eyes humans' history is intertwined with the demons. Maybe they were all prosecuted by humans at some point... It seems like the demon has a goal and isn't just infecting people randomly.
Also, maybe he isn't the one infecting people at all ? I mean why didn't the MC tell her dad "hey remember when I told you that the butler and the captain and other people are infected from coming in contact with a demon ? Well, I met him and he is a red hair"...and described everything about him ? She let that incident go which leads me to believe that despite her not liking the demon, she doesn't think he is all bad/responsible for infecting the people in the castle. After all, she was all serious about the subject before and warned emperor so I doubt she would stay silent after meeting the culprit.
My theory is that: There is another demon infecting people in the castle (maybe even a demonized human and not an existence that wa born a demon) and this demon that she met was sent to get rid of the demon that is responsible for everything (maybe because these actions aren't in the best interest of demons)