kasan doesn't want to leave and it doesn't matter what he wants he fell in love with the person that killed his whole family the brother just wants revenge for his lost. All though I want the main characters to be together you can not look past or forget what happened to kasan family I'm sure if you was in the brothers place you would do anything in your power to kill the murderer of your family even if it hurts your lil brother feeling
I haven't forgotten WHY the brother wants revenge. But would you use and sacrifice your only remaining sibling to get revenge? Without their willing consent? Because the way the brother is going now Kasan would be likely to pay with his life sooner or later for that revenge. Without even a guarantee of success.
As to Kasan not wanting to leave, you could argue that he is suffering from Stockholm Syndrome. He didn't even know Igen is the killer to begin with. That he's become used to his bondage by now. Not saying it is actually so, but I could much more easily sympathize with a character misguidedly taking him away for his own good, than his brother using him for revenge. If his brother wants revenge, fine, but he should ignore Kasan's existence except to save his life.
That's why he had the women tell Kasan to leave and walk around they can't blame him for something he wasn't around to do and he will be with the King and his assistant so they couldn't possibly kill him and sense the king clearly likes him he wouldn't possibly let the other kill Kasan. That was the whole reason the woman talked to K-san in the first place and most likely she said something like "oh I feel sad for k-san being locked in the room all day" and the King overheard her you just assume big brother will "sacrifice" his brother without learning the whole plan most likely doing the confusion he will snatched his brother away.
Also they did try to rescue him but he got caught
Can already sense the trouble brewing. And I blame that woman. Neither she nor the brother care one bit about what Kasan actually wants. If he doesn't do as they wish, they will try to manipulate him or force him any way they can, while certain they're justified in doing so. I would have far more sympathy with these two if they tried to rescue him and get him away.