This particular fellow certainly seems
easy enough to tag.
It sounds like goofy conspiracy theory, but I find myself questioning the role of not just mental illness, but mental illness medication itself in these shootings. Even the adds for lots of those psychoactive medications contain disclaimers about "may cause homicidal/suicidal urges'. Surely most of the hundreds of thousands of users of those medications get the intended benefit, but what if the cost is that they also push a few nutcases over the edge? Not only do we need to consider throwing more resources into mental health, but we need to consider our culture of treatment, beyond the modern standard of "throw some pillz at it".
An extremely quick & lazy google search reveals a semi-interesting take on the subject, from the journalistic powerhouse, the Los Alamos Daily Post:
A Brief History of Psychotropic Drugs Prescribed to Mass Murderers | Los Alamos Daily Post