Michael Goodwin of the New York Post has a good article1 about defending against mass killings.
I think it is time to focus on paying attention to who is dangerous, rather than dispossessing people of things. We dont want to give up on the Second Amendment, but we dont want to bury our heads in the sand about wanton killing either.
One would think if a friend or family member or doctor of a person refers a person, or a member of an organization2 refers another member or the organization itself, to a law enforcement agency as dangerous, the agency would take it seriously. The agency should assign an agent to identify and monitor that "potentially dangerous person" (confidentially, since it might be a referral by a person wanting to prevent a loved one from going off the rails, or it might be a malicious referral!). If the agency determines after identifying the person and some fixed time spent monitoring the person, that the person advocates no more than defensive use of violence, then the monitoring can be stopped. At this point, the person or organization should be notified that they were/are being monitored, but no further information need be provided. After a larger time, the monitoring should be stopped in this case. If the agency sees offensive use of violence is being advocated, the subject becomes a "dangerous person" and monitoring can continue, and no longer needs to be confidential. Law enforcement referrals for monitoring should only start with specific signs of belief in or use of offensive use of violence. If the agent/agency decides that offensive use of violence against civilians is being advocated, they become a "potential terrorist" and an application can be made to a court to declare the person or organization as a "terrorist" and an order can be sought to spy on them and/or to seize any weapons in the person's or organization's possession and/or to take other steps to alleviate the danger. If the agency sees intent to commit violence then they can act immediately to seize all weapons (temporarily), then refer the whole matter to court to make the seizure permanent and/or to take other steps.
And yes! a seriously mentally ill person should not have access to weapons! But in a PC world, a different thinking person would be declared mentally ill, so only their doctor should decide if somebody is sufficiently mentally ill to be barred even without advocacy of violence - and they would have to be revaluated periodically to stay on the barred list. And yes - there are levels of dangerousness - so the list might be divided into classes of barred weapons based on rate of kill, and devices to convert a weapon to a higher rate should be treated like the higher rate weapon. And yes - we need ways to stop violence quickly when it does happen.
One would hope that most of this is the required process already, and that we just need to make sure it actually works that way. It should be an actionable mark of incompetence if this is not happening.
The one existing thing I would change in the law are the laws making it a crime to lie to the FBI (or to any government agent). I do not think it is healthy to start with the premise that the citizenry needs to be forced to cooperate with the FBI or a government agent - in fact I believe it is much better to make sure that government derives power from the willing cooperation of at least some of its citizens. The only places where lies by themselves should be criminal is in a court of law, when they are used to defraud or libel or evade reporting requirements, or when a government agent is testifying to an oversight body. I really think lying is a natural part of being human, and lying by the layman should be explicitly protected as a way to enhance "freedom of association", "freedom from self incrimination", and "freedom of speech". Otherwise it gets too risky, and we simply wont speak up when we see something wrong, especially in a world where "reality distortion fields" and "virtue signalling" are real. Similarly, the FBI should be allowed to issue preliminary "fact check" opinions.
We also need to take away the retroactive and de-facto life sentence tools that enable lynch mobs. But that is a discussion for another time.
PS - Did I say "gun" anywhere but here?
 https://nypost.com/2018/02/17/its-time-to-believe-tipsters-when-they-warn-us-about-maniacs/  including things like chat groups.