I think the show could have been redeemed up until Raphtalia grew up. A story where Naofumi learns to stop being a terrible person while raising Raphtalia could have been a decent story. You could have him eventually start opposing slavery, the kingdom, etc, as he became a better person. Once she became Naofumi's voluntary slave waifu, the show was no longer redeemable. And than once Filo appeared it just got worse and worse.