
It wasn't illegal to buy his stepfather's freedom, that's even what old man said to Muyeon. If he wants his stepdad's freedom he has to do dirty work for old man. Muyeon saved all the money he got from the missions for that purpose, but his 'comrades' spread the rumour that he was saving money so he can buy his stepdad for sex. At some point those 'comrades' decided to rape his stepdad in order to mock Muyeon because despite collecting money they fked stepdad before him. While doing thise awful things to stepdad, they probably told him how Muyeon saved money to buy him and that he was just as gross and awful person.
Illegal part happened when Muyeon killed them all, since you should not kill because of mere whore. Your 'lust' should not make you commit crime again your owner I dare to say. Because that old man practically owned them all. And he was just playing with them and mocking their despair.
As for the part about lust. Yes, Muyeon lusted for his stepdad's body, every living grown man with enough hormones would. But he also loved his stepdad. Because the love was stronger than lust, he restrained himself and cherished stepdad's body. He was aware of lust, but oppressed it with his naive feelings of love.
Why he slept with immortal creature that is using his stepdad's body as vassal? Because he loved the person, he loved a soul, a gentle character of his stepdad. Now when that soul is gone, he cannot hurt it. Immortal creature (sorry, I forgot his name, that's why I'm calling him like that) doesn't know what love us, it's not his stepdad, that sweet and warm person from his memories, and also, that body now can endure pouring lust that was kept in chains for all those years. He's doing all those things he always wanted to do to the body, and since there's no soul he loves and respect so much in it, he can unleash the beast and lose his senses in the lust. That clearly portraits hiw different love is from lust. He doesn't love immortal creature, hejust want to feel warmth of the body whose soul he loved. It's his own despair.
Sorry for such long comment, hope this explains.
As for the backstory missing, I agree, we need explanation from where they came, how srepdad became prostitute, how Muyeon was in his care.
I also hope, if we get backstory, it won't be trope where stepdad hated Muyeon from the start and wasn't actually good person, so Muyeon's efforts were in vain even more and all that used as plot device to pair up him and immortal creature as lovers (╯°Д °)╯╧╧
...I'm confused
they keep talking about lust in the dialogue but I read the flashback and all he wanted to do was make money to free his stepfather from the horrors he was being subjected to as a whore? was that what was illegal? and they both got punished for it....? (boy that doesn't make sense either)
okay sure in the present time he is fucking his stepfather's body, but why exactly did the stepfather hate him so much? he kept calling him a dog and whatever in the flashbacks but what did the man do exactly?? he killed some men who were r@ping him
did I miss somethin?