Wow, Kokushibou really got the quintuple whammy of envy, huh? Surpassed in a field he devoted himself to by somebody who didn’t care about it at all, surpassed by someone younger than him, surpassed by his twin (a circumstance where it’s very easy to see the other person as being not so different, and being in their position as possible if you were born in a different order), surpassed in the eyes of a parent who thought his worthiness was the most important (and may not have even loved him), even surpassed in strength of character.
I don’t think this is a redemptive story like some of the other demons, but it’s definitely a tragedy of circumstance. Kokushibou had, I think, a normal, human strength of character, which was strained by the reality of Yoriichi’s abilities until it bent. Sure, if he was a kind, empathetic person like his brother then he might have been able to take his sense of self-worth being torn to shreds in stride, but c’mon, we can’t leave everybody but the Tanjirous of the world out in the cold.
Plus, the implication is that he dealt with his envy by himself, with no-one to support him and heal it besides the person it targeted. Being able to come out of that on your own is rare, which makes Kokushibou an ordinary human being, and his story a tragedy. Does it redeem him? Ehh. Not nearly as easily as Akaza and Rui, who made their mistakes in the heat of a moment rather than in a slow buildup over the course of their entire lives, but it puts him in a gray area that can be empathized with.