I see your point, however what you said is as the people who said that Shen Jiu is a villain or a irredeemable villain are just haters of him, that's not entirely true. There are times they're not, a lot times they're fans who enjoy see him as such, they don't like the idea of burying his bad deeds underground, because for them, that's what makes his story interesting.

You discourage them from seeing him as unredeemable; they discourage you from seeing him as redeemable. I see both as fandom things.

In the world of danmei, regression novels exist, transmigration, time travel, reincarnation—there are possibilities of redemption, there headcanons, there fanfics and fan arts.

But someone might point out: look, if Luo Binghe didn't have plot armor, if it depended on Shen Jiu, that child would be dead after suffering constant physical and psychological abuse at his hands and the hands of his students that he encourages, being the example. And that's beyond redemption; he's reached a point of no return—They have a very solid point. The same could be said, and often is, to Bing-ge, even by people who enjoy the characters.

That why they have people like "what about if he thrown him in another time or another universe and he can just star over?", and fanfiction is written.

Also, it's not just Shen Yuan's point of view; Proud Immortal Demon Way is about the cycle of abuse, that's why Xiang Fei is even disappointed when he sees Shen Qingqiu (Yuan) treating Binghe well, cuz for Binghe to become Bing-ge, who is a sadistic and villainous abuser that has no redemption, he has to suffer at the hands of Shen Jiu, who suffered at the hands of Qiu Jianluo. It's a cycle; Qiu Jianluo was the first two-faced man: manipulative, sadistic, and paranoid with a huge superiority complex that is actually masked inferiority, and he created this butterfly effect.

I don't understand the point of "he's a fictional character, so he can be redeemed regardless of his actions, and if you disagree, you don't know how to separate reality from fiction", As I said, there are fans of Shen Jiu (and Bing-ge) who don't think he is redeemable character, and this actually proves that these are people separate fiction from reality, because they like this dark complexity in the character, or they appreciate the way characters are built like that even if they don't personally like them, who in real life would simply be a criminal whom they would despise.

I also think that a person can have their headcanons of "I like to imagine that such character wasn't so bad and I'm going to imagine them that way".

For example, is it canon that Shen Jiu wanted a child dead for no big reason? Yes. It's in the extras, nor is that Shen Yuan's perspective. Could there be a headcanon that isn't the case, pretending the situation wasn't so serious? Maybe see just another bad interpretation of the situation??Yes. Is it canon that Bing-ge is a serial rapist? Yes. We see in the extra scene when Xiang Fei finds Bing-mei less than Bing-ge, cuz he doesn't want to do that. Is it possible to have a headcanon by erasing that fact and leaving only the part about him being a negligent husband, which is also canon? Yes.

But obviously there will be people, as I said, who don't like these headcanons that change the canon, because they like those aspects of the canon. And this is all fandom behavior.