But a guy with mental instability can't kill dozens of people as easily without a gun.
The causal factor of death is bullets from a gun. Take the gun away and you have a totally different situation.
Imagine a baby grabs a knife from a drawer. What do you do? devise a treatment scheme to make the baby not a baby anymore or take the knife away?
In The Anatomy of Human Destructiveness, psychoanalyst and social philosopher Erich Fromm described extinction fantasies as an aspect of “necrophilism,” which can feed malignant aggression. People with a "necrophilous character" are guided by a set of values that glorifies death and demolition.
As Klebold struggled with a sense of failure, equating suicide with tranquility and escape, Harris aimed his hatred outward. He attended closely to stories of prior school shootings. The way these young men reinforced each other’s dark side is a striking factor in their “mission.” Each time they worked on their plans, laughing over who might die
https://www.psychologytoday.com/blog/shadow-boxing/201207/mass-murder-motives https://www.psychologytoday.com/blog/shadow-boxing/201207/mass-murder-motives
James Knoll IV, a forensic psychiatrist at Upstate Medical University, part of the State University of New York, has written that in such a narcissistic culture or subculture, affronts to self-esteem can be equated with threats to our very survival and that the typical response to such narcissistic injuries is a desire for revenge.
Mullen described several traits and historical factors that these individuals had in common. In particular, they were bullied or isolated as children, turning into loners who felt despair over being socially excluded. They were generally suspicious, resentful grudge holders who demonstrated obsessional or rigid traits. Narcissistic, grandiose traits were also present, along with heavy use of externalization. They held a worldview of others being generally rejecting and uncaring. As a result, they spent a great deal of time feeling resentful and ruminating over past humiliations. Such ruminations invariably evolved into fantasies about violent revenge. Mullen noted that the offenders seemed to “welcome death,” even perceiving it as bringing them fame with an aura of power.
http://www.jaapl.org/content/38/1/87.full
You are taking one variable and drawing to a conclusion that if this variable is removed, then all will be better. Not very scientific.
Studies show that these same individuals will kill by other methods.
Columbine was intended not primarily as a shooting at all, but as a bombing on a massive scale