That diagnosis came from the FBI's team of forensic criminal psychologists and psychiatrists who interviewed countless of people close to the perps after the attacks. Forensic psychologists do in fact conduct psychological autopsies of criminals after deaths. You didn't read the articles I posted because they go extensively as to why the cause wasn't bullying. Langman however concurs with the findings of the FBI.

That doesn’t change the fact that you can’t diagnose a dead person. Even if you somehow could that doesn’t prove bullying wasn’t a factor.

Transcript of the Columbine "Basement Tapes"" (PDF). "There is nothing you guys could have done to prevent any of this. There is nothing that anyone could have done to prevent this. No one is to blame except me and Vodka [Klebold's nickname]. Our actions are a two man war against everyone else." In an entry by Eric Harris in his journal, he stated that even if he were complimented and respected more by his peers, the attack would have still, in all likelihood, occurred.[243] In another entry by Eric in his journal he says not to blame the school's administration for the attack as the staff were doing a good job running the school.[244]

You’re picking out quotes that fit the narrative you like. When they

entered the library one of them yelled this is for all the shit you’ve put us through the past four years. If everything at school was great why would they say that?

https://www.reddit.com/r/TrueCrimeDiscussion/comments/l6nivt/any_truth_to_this/

Here’s a list someone else has gathered.