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Yeah, from the start I don't have a good feeling about him either. Like he is painted as the kind one that his caring and understanding, but he is also the one that pick the sacrifices and isn't phased by them being eaten. He is also the one that stroke a deal with Monster Mugo, that ended with the need of a sacrifice. As if Main Mugo was purposely kept out of that deal.
So why would Old Mugo spare Ja-Heon. It is not like he grew tired of all the killing seeing he had no problem suggesting to ask for another sacrifice. He is also not really concern about starving the Monster, even though he seemed to have a hand in the reason why he can only eat human flesh.
He is seems too fishy and manipulative.
Do you think that Old Mugo choose Ja-Heon because he knew he was the reincarnation of Mugo's lover from before he turn into a God, to purposely weakened the personalities boundaries and erase main Mugo to take over the body?