Okay, it was a misunderstanding about LWJ.
Its true that evil intentions can easily start that mob mentality, and it was even more convenient to do so when there were SO MANY bad points against WWX. His demonic cultivation, the Wen prison escapee, his personality shift, his own Sect Leader unable or unwilling to defend him.
The problem was that there was no one willing to speak on his behalf, and when WWX did speak, he was brushed off easily with some reason or the other. The only time Wei Wuxian attempted to speak up was in the Jin banquet and he realized that their unreasonableness was because they knew and weren't willing to tell him due to their own malicious reasons.
The reason, I think, that no one followed Wei Wuxian when he went to the camps was so they had the ability to pretend ignorance. They already knew that Wen Ning was being being used as bait by the Jins as the Wens had done during Indoctrination Camp. They could infer that many cruel things were happening and no one wanted a part of that.
I think LXC wasn't a knowingly cruel person. He wouldn't be cruel knowingly, but he was unsympathetic, and did not really care about much, others than his immediate surroundings. One of the things that is very common between the gentry is a mentality of "we are different from them". The mentality that makes people look and go "oh we solved the problem, oops we destroyed the farming land? It's not our problem, whatever".
I don't think the cultivations world is that great either, but I think Lan Xichen wasn't a completely bad person either. He was the only one who brought up that Wen Qing was known to have not killed anyone during the Sunshot Campaign, but when NMJ spoke up, he didn't say anything.
The Lans have a mentality of "we are not involved", even LWJ has it, though not in a bad way. LWJ isn't nosy, while WWX very much is. LXC is also not nosy. He wouldn't interfere in the working of other sects or their issues.
If WWX went off into the burial mounds...How is it their problem? He still spoke up for LWJ's sake, probably. But I think he was mostly just not planning to interfere much. WWX kept rejecting his brother, so it was the Jiang Sect's duty to deal with him. But the problem is, he's invested in JGY who keeps bringing the other Sects in tow, and keeps involving the Lans and therefore the Nies.
Whenever JGY calls, LXC comes running. This is not to mean that it is LXC's genuine opinion. But he's kind of stupid, thinking he's helping his A-Yao. Showing him support and preventing people in the Jin Sect from bullying him. LXC had to keep showing more and more support to JGY until that support was what helped him become the goddamn Chief Cultivator, but even then (in LXC's eyes) he was still being bullied. Oh, what a poor, pitiful brother he had, even in the highest position he could acquire, he still had to listen to people insulting him and his mother...
So, I think LXC was incredibly stupid, but also, I don't think he actually meant harm to WWX until maybe WN killed his people (who he brought to show more support to A-Yao!) or maybe even Nightless City (where he once again brought people to show support for his A-Yao).
LXC is extremely hypocritical, because he doesn't care about anything other than JGY, and it seems that way because JGY takes advantage of LXC's character flaw.
Actually, there were more things that LXC knew. Sorry about not being able to tag the actual book conversations. But LWJ said he "heard" about Xue Yang being brought to the Jins, and who else but LXC would he have heard it from? I think LWJ scorned LXC for not taking a proper stance then (LXC: cause JGY was on the opposite side) and claiming neutrality instead of doing what a Sect Leader of a righteous Sect was supposed to do.
Remember that LXC stood against NMJ to save Xue Yang for JGY's sake as well, and the only reason why LWJ would have been able to state what happened to clearly without adding his own suppositions was if he was told so directly by one of the involved parties aka his brother.
Also... I'm not sure what you meant by contradicting his brother in public? The linked post is about LXC dissing WWX, basically.
And honestly, if we're talking about Sect Leader's negligence...That part I think, falls right onto JC, actually. I don't know why that parts is slipping through so much. JC was WWX's defendant and Sect Leader. If WWX does something wrong, then naturally, the Sect Leader must take responsibility and say "he will be punished for his wrongdoing after investigation". It is literally not the place of the other Sects to punish or reprimand one of their Sect disciples. It was JC's duty to tell them to fuck off. The Wens used to discipline other disciples often and it was referenced many many times in the novel, that it invited a lot of scorn.
You don't disciple other Sect's disciples. It's kind of okay if it's harmless ones, or you are in a position of authority over the one you're disciplining, but WWX was the head disciple. The only one above him were the Sect Leaders. No other Sect disciple had the authority to discipline him, not even LWJ, even if he was the romantic love interest.
WWX being that strong without a lineage... was a threat, that's true. But it wasn't the only reason. Remember JC told WWX that if he stood on their side, he was praised and if he was on the opposite, then he would be condemned?
Honestly, someone that's strong...Being a threat was the problem. The lineage was an excuse. It was because he wasn't controllable. If he was strong and he stayed within his limits, if he had married LWJ or stayed in Yunmeng, yeah, he'd still be scorned a bit, but not as much. Out of power, lineage, and righteousness, he had two, after all.
I'm not saying lineage wasn't the reason for them condemning him so heavily. He was condemned in the Sunshot Campaign too. But I think one of the reasons was his personality change showing exactly how uncontrollable he was. He wouldn't even smile and pretend to play along, instead demanding his will be met.
The lineage was always there. He was always the son of a servant. But he was still so fucking smart and charismatic that people would STILL open the doors for him. Not like JGY, who kept repeating his lacking lineage like a broken record. Actually, somewhere in the Guanyin Temple or maybe in the extras, I'm not completely sure, it was said that JGY actually scorned his lineage the most. He wanted his rightful throne, wanted to discard his heritage of being a prostitute's son. Unlike wwx, who embraced that his parents were a loving real couple along with their lineages.
Also, they moved against WWX cause that's exactly where JGY was leading it to? No one actually wanted to go against the Wens. They didn't have enough reason to then, because they thought they would be safe. Against WWX, the first thing they knew was that he was uncontrollable, was the son of a servant and scorned the power of the Sects (going against the Jin and all). JGY raised the amount of danger they were put in because of WWX and viola! Reasons were created.