As noted, many recent mass shooters purchased their firearms legally - which included a background check.
The thing about background checks is that, essentialy the American public was "sold a bill of goods" promising much and delivering nothing real.
Now, I freely admit that when it comes to a Constitutionally enumerated right, having to get govt approval seems to me to be "guilty until proven innocent", but that's just the principle.
When you get into the details, you find that the checks don't stop anyone who really wants to harm others. The checks can fail to prevent gun purchase when they don't have accurate information, or when they do, and disqualifying information IS NOT ACTED ON. This also happens but isn't reported nearly as often.
Also, in order for the law to actually deter someone, it has to have some teeth. Look at how the govt (present and previous adminsitrations) have treated the matter of prohibited persons lying on the federal forms in order to try and buy a gun from an FFL dealer.
Lying on the form is a separate crime and carries a multi year penalty, in and of itself separate from illegally trying to buy a gun. There is no "teeth" when the govt catches these people, what nearly always happens is they deny the gun purchase, and go no further. Prosecution is not actually zero, but it is so low as to be meaningless.
The other point where background check always "fail" is when there is no disqualifying background to be found.
Its repeated endlessly (it seems) when some mass killer "bought the gun(s) legally" as if there were something wrong with that. There isn't, by definition. If they bought the guns legally, they weren't criminals. They broke no laws. They were not prohibited persons. They were, in fact the same as every other law abiding citizing in this country. And entitled to the same rights and treatment as the rest of us.
its kind of like some wacko deliberately running down several people with a car or truck...ever hear anyone remark on how horrible it was that he bought his car legally and oh gosh, even had a valid driver's license!!
oh the horror!!
Many (not all, but many) of these mass killers don't have any records of any wrongdoing to check. Some have things that might be considered disturbing, but when those things don't meet the legal standard for action, NO CRIME has been committed. Kids will say a lot of creepy stuff, often just for its shock value. Millions upon millions have done this and never gone on to kill people.
But what's the proposed action when perceived threats (real or not) appear now? Red flag laws. Seize their guns now, figure out if there's a real threat later. The potential for abuse is horrendous.
Not the optimal solution as proposed (and in some places already enacted) as I see it.
Consider this, if you can somehow keep guns out of the hands of these potential killers, and if you also harden the recognized targets, you might reduce the mass shootings, some, but will you be able to reduce the violence? or will you just be causing the would be killers to choose other methods and other targets?
one of the biggest mass murders in recent times was done with gasoline, and chained shut nightclub doors.
And then there's the greatest mass murder of our modern age, the terrorist attacks of 9/11, thousands killed many more injured, and no gun was used there, at all.
Point here is if you make one tool harder to get, they will just shift to a different tool.
In order to stop these killings you have to remove the killers desire and will to do it. Fear of punishment doesn't do it, most of these killers don't leave the scene alive, or plan to. Fear of a lifetime in prison doesn't affect a death seeking nihilistic assassin. Not one bit.
And lets be honest, that's what a lot of these shooter are. They're angry, frustrated, (not to mention deluded) and are seeking to go out in a "blaze of glory" in their own minds, taking as many as they can "with them" to feed their egos. If the cops don't kill them, they often do it themselves.
Not all of them, of course. several have been taken alive, some have calmly and peacefully surrendered after their murder spree ended.
What's their punishment? prison? IF they are found mentally competent to stand trial, and get convicted. Psychiatric institutionalization if they aren't found competent to stand trial. Doesn't seem like justice, to me...
Its almost enough to consider demonic possession a valid thing....