First off the bat, you can't force treatment on somebody as that is a violation of human rights. You can't lock them up, you can't feed them medicine and if your treatment consists of having conversations with the patient, he/she can simply refuse to talk.
If people do receive help, informing 3rd parties (whether it's the police, parents or something else) is a strong violation of the individual rights of that person
unless somebody clearly poses a threat to themselve or others in which case this is for the greater good.
As for people who're saying, let's just screen everybody and make a list. First of all, you don't want that as you'll end up with things like selective insurance and all other kinds of bad things. Secondly, it is not the governments job to do this. If a parent notices something odd with their child, they should take action. Thirdly, I don't doubt it is relatively easy to 'cheat' such a screening into having a 'clean' repututation, so all in all it seems like quite a useless idea.
I think responsibility lies with parents and citizens as the parents are the ones who have great influence on their children, should notice odd behavoir (granted the assumption that they spend some time with their children) and are generally responsible for their children. As for American citizens, they're the ones who keep on carrying the tools for tragedies like these, keep on protecting their ancient rights and yet when somebody does start shooting up a school, there is nobody who is carrying these 'rights' to stop them.
In order to do something about situations like these I would try to improve mental heathcare where possible while making sure people their rights aren't violated (as in, distributing personal information, forcing them medicine or locking them up). I would inform parents about possible mental illnesses, make sure there is a good information structure about it and teach them to be responsible parents in general.
On the other hand I would regulate guns. The right to bear arms is from centuries ago and made sense back then, however it doesn't make sense today in a modern, first world country. If this so-called 'right to protect yourself' is actually working, I would be interested in stories that read "Heroic citizen saves school from mass murder" or "Citizen prevents rape" or "Citizen detains bank robbers". It's actually likely that they're out there, but the real question that should be asked is would that have been nessecairy if the gun laws were different.
As for personal safety, I would encourage everybody to read
this blog, as the sources he uses are pretty decent. Generally seen, people think they need a gun to protect themselves because other people all carrying weapons as well. In their term, criminals think they'll need bigger and heavier weapons. Well, you can see where that is going! Which brings us back to the issue of guns being easily accessable for everybody and anyone.
If you would remove most guns from public society (I'd would leave guns around for hunting purposes) and create safety regulations, psychological tests, increased responsibility for your weapons (if your kid goes Rambo with it, you should be partially responsible), etc. In order to obtain a hunting weapon, rules similar to those in Japan make sense:
Source
If you read the whole article, you'll find out Japan has one of the lowest amount of gun deaths in the world.
So, TL;DR:
- Improve mental healthcare
- Increase parent responsibility
- Inform parents about mental illness
- Remove the majority of weapons from the general public
- Increase weapon ownership's responsibility
- Do all this without violating human rights