In order to deal with mental illness, it needs to be diagnosed. We don't know that he was diagnosed with anything.
I have to disagree with this, based on my own experiences with kids and those of friends, many of which adopted older children. Mentally healthy people don't go about shooting children.
We call it "alphabet soup disorder." Diagnosing mental illness is a real mess and there are so many differing opinions out there that you can go to one place one day and another the next, and they'll try to call the problem something different. Then there are disorders that are taken away or renamed. There is one being changed this year from one diagnosis with a name to becoming a cluster of different names instead.
It is extremely frustrating and difficult to deal with. Nobody will diagnose a child as a psychopath because it is such a strong "label," but there is a diagnosis out there that changes to that once the child turns 18. However, since the child is then legal, they cannot be diagnosed with it unless they take themselves in and get it diagnosed, and how many do that? None that I know of!
It is a dangerous misconception that mental illness must have a specific name. All that name really does is get you in the door to getting the help the kid needs and trying to figure out exactly what it is. Sometimes the diagnosis isn't made until they try different meds and see what works. Sometimes, they don't nail down a specific name at all.
Then there's the stigma and disbelief of everyone around. That woman's story? That's a story I've heard over and over. There's a story in the Huffington Post called "I am Adam Lanza's Mother." I highly recommend it. There are many parents out there who spent years screaming for help, many who went bankrupt and/or lost their health and job and even families trying to help these kids heal without support from others, yet they are still blamed for the child's behaviors. Nobody wants to believe a cute little kid is capable of the deep pain and anger that results in this type of behavior, so the parents of these kids are met with disbelief. Suddenly, they are teens or young adults and something like this happens, and all of a sudden everyone is finally paying attention, but often only to place blame on the parent and other people who loved that child for years and dedicated themselves to trying to help them heal despite the deck being stacked against them, despite having little to no support, despite having nobody who GETS IT. Often, the kid has to choose to heal and some do, some don't. We can't force it. We can only do our best to guide them to make the right choices.
So yes, there are mentally ill people out there that aren't diagnosed. There are people diagnosed with something they don't have just so the doctor will have grounds to try different medications to see what might work. Sometimes, having the medication work changes the diagnosis. Sometimes they say "oh, such and such is helping, but we still don't know exactly what it is." Or they know exactly what it is, but are not allowed to legally diagnose it because the kid is just a hair's breadth away from fitting the exact criteria to be diagnosed.
The world of mental illness is pretty messy and tough to navigate. I don't see it getting any easier anytime soon, especially as more and more people become more and more accepting of more and more behaviors as normal instead of wrong and the result of a sick mind.