I don't think it's realistic to think we are ever going to identify one exact root cause of what happened at Columbine. I understand that it would be helpful to know exactly what separated Dylan and Eric from every other kid who was bullied or struggling with mental illness, but real life does not work like that. Doctors are not going to identify the one virus Dylan/Eric had and figure out how to prevent it forever.
Instead, what we can do is address the problems piece by piece.
Dylan and Eric were bullied at school. They were able to access guns far too easily. They both struggled with mental illness. Those things are all relatively common, and the overwhelming majority of people who experience those things do not commit mass violence.
Psychologists who have analyzed Eric have suggested psychopathy. For Dylan, I think they've pointed more to depression and extremely low self-esteem. But there is so much about their mental states that we are never going to know.
They left behind the basement tapes, journals, and things like that. I think the tapes have since been destroyed (not 100% sure on that). Dylan also erased his hard drives. It is entirely possible they talked to each other about what they wanted to leave behind and what they wanted to destroy. We may never know how much evidence was intentionally removed.
And honestly, I do not think we are ever going to know the full story of what went wrong.
I think we should focus on the things we can actually do. Check on your friends. Encourage people to get help. If you see someone getting bullied, intervene. If someone displays concerning behavior, report it. Even if you cannot guarantee that it will prevent something terrible from happening, it still matters.
The one part I am uneasy about is all the systems to report "concerning" behavior. First, I am not convinced they are always as effective as they make themselves out to be. If it becomes incredibly easy to report students for suspicious behavior, you are going to get an enormous number of reports, and staff are not going to have unlimited time to investigate each one. Not to mention there are privacy/due process rights. You can't search someone's house unless there's a warrant and probable cause or whatever.
Second, students can absolutely tell when adults are suddenly watching them more closely. If a kid feels like they are under investigation, they may become even more closed-off, and that can easily backfire. They might stop talking to their friends because they are worried about what someone will say or do. And now the intervention itself can backfire by taking away one of the very support systems that student actually needs.
Like I said, I think there's a lot that we're never going to know about Columbine. Dylan's mother wrote a whole book about what she thinks happened, and she is understandably very biased because she is Dylan's mother. I think Eric's family has mostly kept quiet about all this.