Really going to take a multifaceted approach that focus on treating both causes and symptoms. This isn't a disease that you can just create a cure for to eradicate, so you have to treat the symptoms because the symptoms will always be there. So some of the stuff like single entry way for schools would help here, but that's just like giving someone Tylenol to treat the headache when the real problem is a sinus infection. But treating symptoms never fixes anything, it just makes things suck a little less, and that's why you need to do things that get to the root cause. Whatever is done will never fix it, but it can have a much more profound impact than merely treating symptoms.
A few of the general things I think need to be done are below.
- Gun law reforms - background checks, cracking down on private gun sales in some capacity, illegialization of assault weapons and and high capacity, quick-firing guns (sorry if my terminology is wrong, I'm not a gun guy). People who want to kill will always find a way, that doesn't mean we shouldn't do things to limit the amount of damage or to make it more difficult.
- General law reforms - such as repeat offender stuff as someone else mentioned, for one example,
- Reformation of the criminal justice system - Includes better enforcing the laws on the books. But we need a much greater emphasis on rehabilitation, instead of just simple punishment, so when people are released they are better able to be productive members of society. Ex-convicts generally have an extremely difficult time reintigrating into society, and by not equipping them with the education and skills to be a productive member of society, they'll resort to the more profitable elements of criminal society to get on their feet financially, and those elements are society are the most violent even though they don't get the mass shooting headlines
- Greater investment in mental illness treatment - which would involve better educating people how to identify behaviors that would indicate potentially violent tendencies. If we know that behaviors A, B, and C are warning signs of potential violent future behaviors, we could let someone know that is in a position of power that can potentially help
- Better parenting and education - people can be just absolutely shitty to each other, and it seems normally when you hear about a mass shooting, it's the shooter responding to a group of people being complete assholes to them or because they have ignorant and irrational hate towards a group of people because of their race, religion, sexual orientation, etc. Parents and other supervisors and role models need to do a better job teaching children how to be good human beings and they need to do a better job listening and observing the children to see warning signs of when things are starting to get dicey. Most of these people don't just wake up one morning and go "screw it, I'm going to go shoot up my school." It's a process that we continually fail to ignore and just dismiss odd behavior as odd behavior instead of what it is, warning signs.
- Politicians, media, and other people of influence of all political/belief spectrums need to start behaving and acting like adults whose actions and words can have vast implications on society instead of petulant children spouting out divisive and hate-filled agendas that incite and encourage the worst elements of society. Particiularly with the politicians, they need to start taking real action to solve the problem instead of issuing toothless thoughts, prayers, edicts, and statements about how they will look to fix the problems while doing nothing and continually allowing the NRA to buy their favor with campaign donations.