well we don't know what he's done yet. he always appears with a black haze, and he seems obsessive. we know that jaheon is deeply disturbed and afraid of him, so i can't imagine he treated jaheon well or with consent.
Well they showed us part of his story and told us already how the demon was created. There's a chapter where he says he was kidnapped and put in a giant jar with the demon serpent so... I believe the black haze is the grieve of his dead soul that didn't trascend. His regrets probably and all that spiritual stuff that I don't know the name in English. Jaheon was probably hisnlover for what we can tell so far and was probably killed too by that family and his soul may remember and think he abandoned him or so...
That's so sad. He wanted to meet him again so badly he didn't trascend to the afterlife I cannot hate him. Poor baby.