Seeing as I am a psychiatrist and about 50% of my patients are receiving disability for severe mental illness, I think my opinion may be of higher value here. I will say a few things.
First, we must separate this argument into 2 concerns: homicide and suicide.
Regarding homicide, mental illness is a risk factor but the vast majority of homicides have absolutely nothing to do with mental illness.
Regarding suicide, there is actual evidence for means restriction and gun ownership. Suicide by gun is about twice as prevalent as murder by gun, and it seems to be overlooked quite a bit in these discussions.
There are 2 very compelling hazards that I see when considering these laws.
1. Restricting gun ownership to the mentally ill is an impediment to people from seeking mental health care. Even the perception of this has come up a few times with patients who have asked if somehow my care restricted them from owning a gun or otherwise noted that they restricted the information they shared with out of fear that I would somehow restrict their gun rights. And I know nothing about patients who have had these thoughts and not shared them with me. And of course I know nothing about people who never seek mental health care in part due to this fear. Given the lifetime prevalence of mental illness (including substance abuse) is close to half the population, this perception seems a mighty large barrier to treatment. I have to wonder, which is more likely: someone who seeks mental illness is prevented from killing self or others because their gun rights were removed, or someone whose treatment of mental illness would prevent them from killing self or others but never got treatment because they feared their gun rights would be removed?
2. Is it constitutional to remove 2nd amendment rights on basis of mental illness? This is the argument cited by the house shutting it down. While I find no fault with the argument that we could reliably classify a bunch of people that we would rather not own guns, I find a lot of fault in removing someone's constitutional rights because we think they are better off without them. For instance, no reasonable person would question that removing gun rights to black people would be unconstitutional (nor immoral). But I feel certain it would be more effective at reducing homicides.
Personally, though, I think all of this is rendered moot when I consider how easy it is for my patients to obtain guns illegally.
Lastly, I do want to put a little social commentary here. There is a function here of ostracizing mental illness and splitting it off as the root of gun-related problems. It gives us a fantasies that 1. people that we feel comfortable not identifying with are responsible and 2. we can do something about the problem. Unfortunately, it's not true. At best, these interventions address a minority of the gun problems we have. And people with mental illness are still people whose rights ought to be in tact. It is far too convenient to feel it OK to paint a subhuman picture. It is not dissimilar from seeing blacks as constitutionally inferior in intellect, lazy, having more violent drives, etc. which were still acceptable beliefs even during the civil rights movement.
