@OsoJr nah, I think his vengeance was for everyone (aside from that childhood friend) continuously beating him up (I would argue they kept it to the point where he could still do his job, but considering this kind of "getting a redo" thing always comes after death or defeating the big bad, I have to assume that night was when he were actually beaten to death. So they clearly didn't), treating him as an actual slave. It is a bit unclear whether the guy joined in the beatings, but at the very least he seems perceptive enough to have known for sure that they were happening. And clearly didn't care if the guy was badly hurt after fighting monsters, instead taking time to bask in salutations.
That said, I agree everything is confusing. The translation likely contributed to that, but like you said the story managed it fairly well on its own too by leaving way too many plot-threads dangling, and introducing characters that seemed like they had backstory but none was mentioned (one could almost believe this was an adaptation that was both rushed and axed - actually, I just googled, seems it
is an adaptation).