Every time one of these posts come up, I become convinced that vanishingly few people understand mental illness and its effects on its sufferers, its diagnosis, treatment/management and outward signs of mental illness.
If you are going to talk about categorically removing a right from someone on the basis of mental illness, you are going to have to make DAMN sure you know what that means and how you would identify it. Do we mean that no one with depression should own a gun? That's a lot of people. Do we mean only those diagnosed with violent antisocial behaviour disorders? (think Sociopathy/Psychopathy), that involves time intensive and repeat assessments with trained clinical psychologists and board certified psychiatrists. Just not reasonable. Post hoc saying "anyone who could do this is obviously mentally ill" is not only not true, but isn't useful in any way. We need behaviours to point to before the fact that warrant the removal of a right guaranteed to us (for better or worse, not relevant to my argument).
With this guy, I think we had a history of him being a little unhinged, with episodes of domestic violence etc. Are those warning signs? Absolutely. But let me be absolutely clear, that isn't ever going to be sufficient for the removal of a right promised to a person by the law of the land they live in. You have to come up with a test that is both sensitive (catches a lot of the people who are actually going to commit a violent crime) and specific (doesn't catch a lot/any people that won't).
The way the law is written, our protections on gun ownership are as important as our protections on free assembly, speech, to vote... whether or not you think that is a good thing, that's the world we live in. If you want to approach the control topic from a perspective of mental illness, you are going to have to revolutionize the way we diagnose and manage those mental illnesses.
Also, can we stop lumping in all mental illnesses? Most people with mental illness are actually really vulnerable, underserved and at very high risk of having violence committed against them.