I don't think it is likely that a mental health evaluation would have demonstrated much. The law only bars those who have been involuntarily committed from getting firearms, a diagnosis of mental illness is not sufficient. And an involuntary commitment is a pretty high bar. My Dad is a clinical psychologist who evaluates people for the courts, and he can go on and on about how the severely mentally ill often do not qualify for involuntary commitment. Unless you are actively trying to hurt yourself or others you will not be committed, which basically limits it to people who are either attempting suicide, or assaulting people.

I'd also like to point out that there are a number of issues with the current system besides what you've mentioned that could improve things. For starters, even if this person was involuntarily committed, he would fail a background check, but would still be able to purchase guns online or at a gun show without a background check. There are red flag laws, which allow people to petition the court to temporarily seize guns from someone acting in a disturbing way, which would have been ideal for this case, but these are only in some states.

You can not purchase a gun without a background check online, at least not legally.

Online or by-mail purchases can only be shipped to an in-state dealer, where you must complete a background check.

This has been the case for twenty years.

Neither can you do so at a gun show. The same laws apply.

You can not legally obtain a weapon from a licensed dealer without a background check. Period.

If you derive any notable income from selling weapons, then you are legally required to register as a dealer. Failure to do so is a federal felony.

This is why weapons used in crimes do not generally come from legal sources. The vast majority are either stolen, or illegally purchased by a friend of family member.

Please increase the information to inflammation ratio of your comments.

https://news.ycombinator.com/newsguidelines.html

The point I'm trying to make is that blaming all of these shootings on mental illness is completely baseless. All it does is further stigmatize mental illness, and it does so by conflating people that, in all honesty, are just terrible, terrible people, with people who have mental illness. Combining that scapegoat with the fact that we, as a country, do absolutely nothing to treat mental illness, just makes life worse for those who actually do have mental illness.

OK, please make it more like that next time.