Even Lan Zhan didn't start to question anything until after Wei Ying died. But, he never seemed to wonder at all why Wei Ying started to utilize demonic cultivation in the first place.
I always believed that those people were ruled and led by jealousy. You know the type to get infuriated when seeing someone so self-righteous trying to tell their old morally corrupted ass that they are wrong. Seeing Wei Ying also seemingly so strong and powerful they were also feel threatened by his existence alone. So any small mistakes is focused on and amplify so they can get rid of him and rid themselves of any stains on their believes and ways of thinking. They just can't stand his existence and everything was led by bias and motive. Even LWJ bro has this biased view on Wei Ying. Seeing how arrogant Wei Ying was and coupled with the questionable
Morals he was brought up with in Cloud Recesses. I can see how he got such a skewed conception.
For LWJ, I guess he never questions it because he was not surprise knowing Wei Ying well. Wei Ying is an experimentalist and always curious on different ways which can be done. He was always curious about how to use this power and is open about it. So I guess LWJ sees it as not surprising Wei Ying will resort to demonic Cultivation to be stronger to protect ones he love. But this is just my thoughts on them though
And I don't get why not ONE of them questions stuff. None ever did. Except for the Wen siblings and Lan Zhan. Maaaybe Lan Xichen does sometimes, but not to the end. Almost everyone in this world is so stubborn and bullheaded. With their damn annoying leader Jiang Cheng. God, go die already (not reeeeally... or maybe... xD). With all sympathy I have or had for you, but 16 years past and he still behaves like a spoiled child and grumpy old man in one.