The data from Mother Jones is quite interesting since it includes a lot of different things.
https://www.motherjones.com/politics/2012/12/mass-shootings-mother-jones-full-data/
Only 64 of the 129 mass shooters had any indication of a mental issue before they committed their crimes. And that is just an indication, it isn't even a real diagnosis of a threat.
This is the mental health indication for James Huberty - The day before the shooting, he tried to make an appointment at a mental health clinic.
For Amid Alissi - Brother described him as paranoid; multiple accounts of anger issues from others who knew him. Misdemeanor criminal record for a violent assault during his senior year in high school.
Richard Hawkins - He had been treated in the past for depression and ADHD.
Terry Ratzmann -Neighbors said he suffered from depression and had a drinking problem.
Are you suggesting we take away the guns from anyone described as paranoid, depressed, with a drinking problem or with ADHD? Or should we be forcing anyone with those problems to seek medical care?