Every time mass shootings happen Republicans trot out the "mental illness" canard. It is a classic Republican deflection from the systemic to the individual, always with bad results.
The system in this case is our body of federal and state laws surrounding gun-ownership. The "systems" within states/country enable MASS actions that affect everyone.
When Bill Clinton signed an assault weapons ban in 1994:
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“During the 1994-2004 ban:
In the years after the assault weapons ban went into effect, the number of deaths from mass shootings fell, and the increase in the annual number of incidents slowed down. Even including 1999’s Columbine High School massacre – the deadliest mass shooting during the period of the ban – the 1994 to 2004 period saw lower average annual rates of both mass shootings and deaths resulting from such incidents than before the ban’s inception.
From 2004 onward:
The data shows an almost immediate – and steep – rise in mass shooting deaths in the years after the assault weapons ban expired in 2004.”
newhampshirebulletin.com/...
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That was SYSTEMIC and it had wide impact in both directions.
When you say the problem is with individuals then your "solutions" have to target individuals and finding, in 330 million citizens, those likely to mass-shoot - or those with mental illness for that matter - is really hard ESPECIALLY if you do what you can to limit the "system" from helping. Making people register guns is systemic and helps with tracking flow of weapons and those who have them, as an example. FULLY funding mental-health programs via FULLY funding healthcare availability etc. is systemic and can work in tandem with other systems. Passing laws to actually protect battered women/children from violent ex's is another element. Therefore cutting or eliminating funding from those and other interlocking systems reduces the collective "system's" ability to help. Which is exactly what Republicans/Conservatives want.
Blame individuals - blame lack of school prayer is another variation - lets people spout bs that sounds like good faith efforts to help when in fact they are doomed to fail and are used intentionally to that end.
Republican politicians believe keeping their base in a constant state of fear and rage benefits them (the pols). Mass shootings feed that fear, ergo Republicans literally take actions to ensure such shootings continue. But because those events upset people they get pushed to comment and defend their inactions and laws and thus they respond with the "mentally ill individuals" canard because it diverts attention from their systemic actions which are intended to keep the shootings from stopping.