Spoilers.....
ML is the baby of his master. The bald person in the last panel is his sister, the one that MC dad thinks his first wife killed. They buried her in a shallow grave but she wasn't dead. She lives in an isolated temple as a Buddhist nun. ML is setting all of this up for revenge, he even intended to take revenge on the MC but he is falling for him instead. MC is not the son of a barbarian, he is the son of a servant that the mom was with. ML's sister saw them together, that's why the wife tried to kill her. ML was taken in by someone else and played off as their son to protect him. But the agreement was that he can't have children to inherit now the his 'brothers' are dead. His uncles son will inherit. MC's sister is shit, he thinks she cares about him but she really doesn't. She is sleeping with her servant, she gets pregnant by the servant. ML has the servant killed, exposes the wrongdoings of the father getting them executed. The sister then dies along with the baby. At her funeral MC's horse bolts, since he can't actually ride and the horse was being lead, he can't stop it. He winds up lost in the mountains and finds the temple where the ML's sister is, she gives him a place to stay and he works in the gardens. She doesn't know who he is. Meanwhile ML is loosing his shit looking for him and only finds him because he goes to visit his sister, he takes him back home to be together. That's all the stuff about why he's doing what he's doing, but so freaking much more happens. The dude who tried to rape him in the brothel, tries to get him. The doctor tries to help him get away. ML locks him up and won't let him have visitors and even removes most of the servants so MC get's sad and depressed.
SPOILER
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After the marriage, the ML takes the MC with him to Hanyang. One day, the prince the prince takes an interest on the MC and tries to assault him when the MC refuses to come with him, but the ML manages to stop him on time and badly beats the prince up. The MC ends up hearing that the prince will pardon the ML if he sends someone (ie the MC) to serve the prince. (Note: he didn't really have to since the king was lenient on the ML and even agreed that the prince was pretty much scum.)
So, he goes to the prince's place because he doesn't want the ML to be punished because of him, but he gets lost tho. The doctor sees him by chance and accompanies him to the prince's place. However, before he could even enter the place, the ML catches up and takes him back. He's hella mad to see him with the doctor and otw to the prince's place. At this point of the story, the ML has realized he's in love with the MC and is really possessive of him.
When the MC explains that he intended to go to the prince for the ML's sake, he gets even madder over the fact that the MC would willingly offer himself up and rapes the MC out of his anger, saying sth like "the prince can do this to you and i can't??" yada yada

Spoilers please