Why am I so sad for the Rengoku Killer, I guess I started to understand him when the well poisoning scene was connected with his hatred of weak people.
But still that does not excuse that it is wrong. Fair Death, Fair Fight, Else it woul've dragged on.
Every enemy that Tanjirou killed isn't "redeemed" when they show their backstory. Their stories are told to empathize with. To point that anyone can break, and can lead to a destructive path unless they see it. And to ultimately show that even the worst demons are still humans.