is a otherworler, that is know, his skill didn't kill other people to activate buff, his skill steal other people skill AND their exp, there was some limitation, like it seems he had to talk with them or at least have physical contact when they were using the skill for him to steal it, that was why his allies always died, he killed them, seems like the exp was stolen from simply being around the target, too OP so is likely that was a sinergy of multiple skills he stolen, probably he only got the skil lthieving and then got exp drain from someone else.
MC had to fight him as the asshole was hired by the elves, MC lost the fight as he was too limited and was losing his levels, so he blocked him and made ally kill them both.
MC survived as the ally was some kind of demon that erased sins, sins being cheat skills, it super killed Reaper but spared MC, but not first putting more limits on him, like lifespan drain if he used his "sin", but it looks the author changed idea mid-chapter and made that the "sin" wasn't his own cheat, it made so MC could use the sin delete ability too, but it used his own life, aside that all as normal.