I think a lot of the people in the comments seem to have forgotten that in most cases the demons were originally victims themselves. The ones that actually retain something of themselves are the exceptions and even Tamayo was shown to have killed and eaten her own family before regaining her sense of self. Tanjirou's last remaining member of his family tried to kill him against her own will after turning into a demon so he knows as well as anyone about the human suffering inside the demon, which is why he isn't just fighting to kill them but also to save them, even if that's only letting them die in peace.
It's something that's been shown almost every time Tanjirou's killed a demon up until now so I don't know how anyone has managed to miss it but this flashback isn't to redeem Akaza, it's the process of showing Hakuji's soul being saved/freed from the demon blood.
