In many cases, "mental illness" is nothing more than a cop-out aimed at reducing/eliminating the concept of personal responsibility in one's life.
From what I've seen, it seems that this guy in El Paso was just a loser -- plain and simple. How he got to be a loser isn't clear to me. How a free society deals with losers is the real problem here.
The good news is that this isn't a new problem. Societies have dealt with this question since the dawn of human civilization. The bad news is that "fixing" it usually involves measures that nobody wants to admit are even being implemented at all -- like totalitarian governments with death squads, brutal punishment for crime, and what might euphemistically be called "ethnic cleansing" in some cases. Here in the U.S. we try to sanitize the process by paying Planned Parenthood to do it for us ... even as we look the other way while they make money on the side harvesting human organs in the process.
They used to be sent "over the top" into a hail of machine gun fire. Any who hesitated and/or refused were summarily executed by either war police and/or officers.
War, plague, pestilence and famine used to keep the human population in check. The bald ape literally breeds like rabbits; with no effective predator extant.
The advent of nuclear weapons and other forms of WMD have rendered large-scale population cleansing efforts moot. So, the extremely dysfunctional & defective, equipped with access to rapid fire arms is able to wreak its vengeance on society at large.
How it's managed/controlled I don't really know; perhaps gene therapy and or DNA analysis?