I don’t think he’s very fair? First of MC wouldn’t have to be going through any of this? Technically it’s the ML fault. Also ML didn’t really seem to have any sins needing repentance of, unless I guess you count ignorance? King Yama seems like he’s just playing with everyone for his own amusement… Using the young masters face is just a way to torment the ML. Also he scratched up MC and it’ll last into his next reincarnations?!? Like if your the king of the underworld hurry up and send him off peacefully?
about the sins needing repentance, I think it doesn't necessarily have to mean what he's actually done wrong but what he so strongly believe that he's done wrong. He blames himself for the death of everyone in the house that day and then later young master's death so he probability thinks he needs to be punished for that. We also don't know what his life looked like after getting to that pub or whatever so I hope it'll be shown in the next chapter.
And the scratch on Seowoo... I actually like that it will be with him in the next reincarnation lol fangirl in me is like wow maybe they'll be reincarnated together and maybe mr reaper will recognize Seowoo by it or something ( ̄∇ ̄")

King Yama seems like a very fair entity even if he has a mischievous look to him. I feel so sad for MC and ML, i hope they can be together forever