@SpookyPants
That doesn't make any sense! Why would you use a weak party member as a sacrifice for the hero? Wouldn't someone with higher expectations like a arch mage or a sword saint be better? And how did they came with the conclusion that the hero is full darkness inside him that needs to be purges? It is because he gave a sour look towards the party cause what happened last night?
Why are you thinking harder than the author? Its a revenge-isekai, the trope here is make you simpathize the main character and hate the antagonists, creating engament because readers have the expectations that, in the future, the protagonist would revenge, in the most worst and awfuls ways, the villains. Always remenber: the most successfull forms of entertainment, like sports, blockbuster movies or porn, use of our basic nature and instincts. This is no exception.
edit: God damn, the author even changes the hair of the protagonist to a half-white-half-black. Arifureta vibes.