Where I live (not the USA) if you have a mental disorder of some kind (Severe Depression, Suicidal thoughts, Bipolar, Rage, something that's keeping you from functioning normally, etc) you are monitored and visited at your home twice (or more) a week. The distinction though is that we have free health care so treatment is available to everyone. If you have Autism or Asberger's you get help as well but it's not monitored in the same way. The goal of all these programs is to make people function at their maximum capacity. Job training, internships, education, counseling, treatment, medication monitoring, etc. When someone is deemed unable to be a productive member of society they are put on early retirement and treated for the rest of their lives. You can be a teenager and retired if you're fucked up enough. You might work to keep busy but never as a fully functioning adult. Maybe just a soup kitchen a couple hours a week or folding laundry at a retirement community. With any luck treatment will get you functioning.
There's very little chance of someone with a mental disorder being able to stockpile a bunch of weapons and build up a plan for mass murder. They have someone in their home twice a week and they don't just sit on the couch and talk. They cook together, talk, read, do dishes and laundry together. They work on whatever problems the person is struggling with. Boomerang's sister would be monitored regularly. Inherited weapons would never stay in her possession.
In the USA people are often just left at home and their family takes care of them. If you want something done you need the courts involved as far as I know. You can't just say that someone's crazy and have the state come and help them. The state isn't interested until the courts have been involved. So if you want to force someone to get help because you think they're a danger to themselves or others you have to call the police, have them arrested/taken to a hospital, get a *limited number of* hours hold on them, and then go to court. Imagine doing that to someone in your family.
Nothing is fool proof though. That lunatic in Norway probably had a system for help at his disposal but he killed 77 people. I know that was more of a political far right attack and he was deemed not insane but lets face it, there was something wrong with him.
Another thing that does complicate things is that guns are a RIGHT and not a privilege unlike getting a driver's license. So our culture has to come to grips that some people (convicts) forfeit their RIGHT to bear arms.