He's not entirely wrong, mental illness and lack of proper investigation into reports and tips are very real causes of why these things are happening. When was the last time an attack like this happened that we didn't find out later that the authorities had been tipped off before the attack, or that someone saw something suspicious but didn't do anything about it?
I am not saying mass shooters are mentally stable. I am saying that their mental instability is not representative of mental illness as a class. I am saying that there is no evidence that banning guns for people diagnosed with mental illness will be more effective at stopping mass shootings than banning guns for the same number of people chosen at random.
If a mental health provider receives information that a person is fantasizing about commission of mass violence and has access of means to do so, that is appropriate information for an intervention. But the sentence is even better if you simply exchange the words "mental health provider" with "any person".
I can tell you, as a mental health provider, I have had a few patients disclose to me that they have been dishonest in their treatment or avoided seeking help due to fear of being reported to have their right to firearms taken away. One time, that came from a person who neither owned guns nor planned to do so. The threat of this rights restriction
has caused harmed to people. The benefit of this threat is both unproven and unlikely.
Certainly we need to do better as a society to recognize, diagnose, and treat mental illness. Doing so probably would reduce mass shootings. But that should not be the aim.