Kinda echoing my other reply. I just don't really see what is to be taken away from a documentary on an event like this other than re-opening an old wound. I don't like when media gives these assholes any more attention. Not always, but often I see films like these as Hollywood just using material to make a new film and nothing else. If the document focuses on the victims and their families than great, I hope it does.

To answer your question more directly take for instance a documentary about 9/11. To me there was a lot that went wrong that led up to that that. There were a lot of unspoken heroes and individuals who made sacrifices to save lives. And ultimately it lead to a war for years in the middle east that changed the course of history. There is a lot to be learned and taken away from a documentary like that. Many things you might not have otherwise known. We can learn where we messed up and maybe how to better prevent that from ever happening again, and we have.

Maybe I'm wrong, but for a documentary on a shooting like this, imho there just really isn't. It'll just retell a sad story of an unfortunate event due to a sick individual.