Well that's interesting. The demons are under non-lethal orders at worst and the hero has a sliver of a conscience at least (possibly also lied to, the sage fucker took way too much enjoyment in crushing the MC for him to be trusted)
Actually, with the demons under those orders, either it's something truly horrendous that needs an extinction event quantity of souls to achieve, or the purpose of the divine Oracle that made the party in the first place is morally suspect...