@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).
@feha idk I think it would be contradictory to the Hero's personality just to dismiss MC's beatings because his teammates deem him "useless". The hero definitely isn't utilitarian, but he still has a proper sense of justice. Also, wasn't it still only the sword girl who was abusing him? The witch seemed more or less just apathetic towards him.
it covers 3 volumes of the WN, done it in a really rushy way... The guy caria kills in the end, for example, is a character (lizard) and play some part in the history, Filaret is more "needy", he tells about the childhood friend to caria...
This guy not only went back in time and killed the hero who wasn't bad he picked up his harem instead which was mean and arrogant. He also was jelous of the hero for being lovers with his childhood love but in the end he didn't even meet her after killing her lover. She's also probably effected from the result of this invasion he changed the history