While I agree re: "accepts that SJ turned out horrible because of him", I do wanna comment on the idea that Yue Qingyuan believing Shen Jiu does wrong even when there isn't proof or that he didn't try to curb Shen Jiu's behavior, at least in the beginning.
For instance, in the brothels, Shen Jiu tells Yue Qingyuan to his face that he's there for the reasons everyone else has also assumed: that he's there to have sex, in the place that sells sex.
âIf you donât tell them and I donât either, whoâd know which sect weâre from?â Shen Qingqiu countered. âCang Qiong Mountain is Cang Qiong Mountain. Tell me, which of Cang Qiong Mountainâs rules decrees that its disciples canât visit this place? Cang Qiong Mountain isnât a Buddhist monastery or Daoist temple. They might be able to control everything else, but they canât stop me from looking for women! If Shixiong finds it shameful, he should watch that mouth of Liu Qinggeâs.â
This isn't a situation where Yue Qingyuan was told a rumor and then immediately believed it. He found Shen Jiu in that brothel after learning that he'd badly beaten up a disciple who'd found him there, and walked in on the middle of him and Liu Qingge having a very public brawl about it. Yue Qingyuan is 'in a rare fit of anger' over it, shouting at him even, only cowing after Shen Qingqiu's speech about looking for women.
Even so, Yue Qingyuan does still ask him directly about why he's there:
âIf you truly are doing well, why do you never spend the night on Qing Jing Peak?â Yue Qingyuan asked, following on his heels.
Shen Qingqiu gave him a dark look. He knew that Yue Qingyuan must be thinking that he was being excluded by the rest of Qing Jing Peak.
Yue Qingyuanâs guess wasnât unreasonable, but he was genuinely wrong this time.Â
Both the readers and Shen Jiu know that this isn't true. But Shen Jiu pointedly never corrects him, because the alternative is to admit he's there because he can't stand sleeping around men anymore, something he won't admit because it's embarrassing.
For Yue Qingyuan to know this though, he'd have to practically be a mind-reader. He has no idea that this is all a result of how Shen Jiu found comfort and safety with Qiu Haitang; all he has is what he's seen and Shen Jiu's own words.
As for killing Liu Qingge...
The thing Shen Qingqiu most often said to Liu Qingge was: âLiu Qingge, Iâll definitely kill you one day!â
(...)
As Shen Qingqiu had turned a deaf ear to his every plea, Yue Qingyuan found it difficult to speak further. After all, he couldnât straight-up tell him something like: If you hadnât used every means to ambush Liu Qingge after the meet in order to teach him a lesson, the two of you wouldnât fly into a rage at the slightest contact now, nor would you loathe the mere sight of each other.
This was a reputation Shen Jiu built entirely by himself If someone goes around threatening to kill someone, if he is violently ambushing them repeatedly, and then suddenly that person dies violently when he's the only other person nearby... Literally what else is anyone supposed to think?
We don't know how much, if at all, Shen Jiu defended himself after this or just what sort of investigations went on in the sect -- but we know that he was still peak lord for 8 more years until Binghe returns, and Yue Qingyuan asks him bluntly in prison:
Suddenly, he asked, âDid you really kill Liu-shidi?â
Shen Qingqiu didnât want to look at his face at all while he answered. But he couldnât help but lift his eyes, giving Yue Qingyuanâs expression a quick glance. He froze for a moment, then abruptly yanked his hand out of Yue Qingyuanâs grip and sat up.
âYou repeatedly said that youâd kill him one day,â said Yue Qingyuan. âBut I never thought that you would actually do it.â
Nearly ten years, and he still doesn't seem to believe it! But that's also ten+ years of watching Shen Qingqiu attack his martial siblings, beat children, and now stands accused of even more murder and arson on top of it all.
It really feels like Yue Qingyuan is stuck, both in the story and within fandom. If he doesn't believe any of it, he's being naive and stupid, because Shen Jiu did do a lot of these things, but then he believes any of it, he's betraying his Xiao-Jiu. What is he supposed to do beyond asking Shen Jiu himself, who never gives him a direct answer either way, neither defending himself or admitting when he was in the wrong. And this all makes sense for someone like him: Shen Jiu fully believes that he's scum by now, that there's no defending himself as much as there's no reason why he shouldn't have acted the way he did.
But Yue Qingyuan apparently still wanted to hear it from him because like you said, he is used to Shen Qingqiu doing wrong and carries a ton of guilt, but he's also been holding out hope for decades that maybe if he was stubborn and patient enough, Shen Qingqiu's anger would dissipate the way it used to when they were kids. That's why I think Shen Jiu's line "There is no âif,â no âin the beginningââthere was never any chance of redemption!" finally crushes him.
It's part of why Qijiu works so well as the mirror to Bingqiu. Binghe's chasing Shen Yuan around, wanting explanations and confused why he's being treated this way, before deciding he can't trust him anymore. Meanwhile, Shen Yuan's big moment is realizing that similarly believing there's no âif,â no âin the beginningâ, and no chance of redemption from sending Binghe to the Abyss has only ended up making things worse, because Binghe could only draw his own conclusions from Shen Yuan's silence.
Shen Yuan, Luo Binghe, and even Yue Qingyuan himself all learn the lesson about reaching out even when its deeply upsetting or shameful. Shen Jiu never does until it's too late.
So ultimately, I don't personally think it's necessarily that Yue Qingyuan is slow to believe Shen Jiu. If anything, I think he believed that his capacity for kindness and loyalty - the reason why he thinks Shen Jiu and Liu Qingge could be friends -- for far longer and too deeply than he believed in Shen Jiu's growing cruelty and self-destruction. And even when he does realize it, he still tries to save him. :']
