Another thing to consider is whether this opens the door for another slippery-slope situation like the bans on "high capacity" magazines (10 vs 7 rounds, etc.). Today, maybe the categories of mental illness that would preclude one from legally possessing a firearm might include one set of conditions, but as more incidents happen, the government may start adding to that list. For example, that shooter had ADHD; therefore, people with ADHD should not be allowed to own firearms.
What would be the process for adding (or removing) conditions from the list of mental conditions that are considered "dangerous" and would therefore restrict firearms ownership?
Will such a mechanism be used to gradually whittle away at the gun-owning population in a way that is systematically designed to disarm the American citizenry over the course of the next 20, 30, or 50 years?
And what about "fads" in psychiatry that lead to over-diagnosis of conditions like ADHD
Another thing to consider is whether the government is justified in denying someone's constitutional rights on the basis of a potential to act, rather than on the individual's actions. In this way, it differs from the stripping of 2A (and other) rights from convicted felons. Here, the government is taking rights away based on something that a person hasn't done and which a person may or may not ever actually choose to do.