First off, he had Asperger's, which isn't a mental illness. Not saying he didn't have severe mental illness as well. Second, we need to consciously uncouple mass shootings from mental health issues. They're a cop-out. They're a way for all of us to avoid the hard stuff, avoid the responsibility of taking care of each other and of our society. They're a way of saying, "Oh, it can't happen in my neighborhood" or "These things are only done by sick people."
How long will the media focus on the Asperger's part of the case? And not on the gun part? If we ever do get any gun legislation, how powerful compared to the other provisions will the denying guns to the mentally ill be? In future mental health care, will there be more suspicion that the sick will be violent?
Guns are inherently philosophical. A recent history of Western philosophy by Economist editor Anthony Gottlieb is entitled The Dream of Reason. Mastering your emotions is basically the definition of philosophy—so long the subject has been written about by thinkers and created new schools of the field. So guns give a literal life and death meaning to the subject. They ask, "Okay, so you say you are a reasonable man and that you can behave without excess destructive emotions. Here I am—prove it. If you fail, will you use me?" Guns are tools of ego or community—they test us on that too. They are empathy tests. Do our problems overwhelm us enough that we devalue other people or do we recognize that our issues aren't as big a deal as our strenghts of community and love and that we can solve our problems, or go a way in doing so, by entering the sea of humanity? A wrong answer will provide casualties.
So: either you can have imperfect people, i.e. people, or you can have guns. You can't have both. And recognizing that there are imperfect people means recognizing that our imperfections are built in, part of the human condition, and not a mental disorder.