Sad to hear of such a thing happening and within your circle....mental health issues are nothing to play with and because teenagers are so erratic to start with, it's often hard to detect.
I personally think one of the major issues of today is the lack of ability to detach from everyone else as social media has become part of people's lives and a complete stranger thousands of miles can have a huge (sometimes negative) impact on another person......and if that person happens to be a vulnerable teenager than the issue triples.
I get the gun thing, I know of people who have many bikes or vehicles and we know we can only ride or drive one at a time, but these people enjoy ownership...I get that..
To put it in perspective from my point of view, would these people have the keys to their GTS Viper or Hayabusa available at all times to their teenaged son? This son who could be egged on by his peers to take one of the vehicles out? We've all seen "Ferris Bueller's Day off" that's what could happen, the scene where the son is kicking the car because he was upset at his father....substitute car with gun...
Having a lethal weapon platform in the home means responsibility of them, locking them up and keeping them from perhaps a troubled teen-one that is hard to detect just as your story depicts...take butcher knife away and add AR15 with multiple box mags and see how many more get killed...
My story for you, I have a friend who is a weapon collector so has all the special permits required, his weapons are all basically in a vault in his basement (actually it IS his basement), he has an alarm system, bars on the windows, a key pad system to get into the basement and his ammo and weapons are locked in separate areas...he even has a panic button in the home in case of a home invasion where the bad guys want to get into his weapon vault. He recently got rid of them mostly to a museum as he was sick and tired of carrying around the responsibility of care-taking for these weapons....he loved them but was losing sleep worrying about them and worried they could end up in the wrong hands. That to me is a responsible gun owner. The good news is he retired and now works in the museum as a curator.