literally, this was happening right under his nose, in his own house. ignorance and stupidity are not an excuse. the fact that he was insane doesn't change the fact that he raped him.
i'm not discounting that he's a rapist but i doubt that that was their reasoning to kidnap him. if they wanted to get revenge, there were many others who looked down on them, raped them, willfully treated them like a baby-making machine, as someone beneath them.
they prob chose muhan bc he was a sol with barely any power or influence and bc he was just there at the right place, at the right time
they should've captured the uncle or other nobles who brought him in as a surrogate... this guy wasn't even in his right mind and barely had anything to do with the years of oppression lunas had to face (so far) (╯°Д °)╯╧╧