I am so sorry, this type of thing is so difficult to deal with for everyone affected by it. Don't dwell too much on the "why" or on searching yourself for what could have been done to prevent this, depression is complex and we are still trying to understand the underlying causes that drive it. Speaking from my own experience, people suffering from depression often also do a very good job of hiding their torment from the rest of the world and it is very difficult to spot and prevent. For some of us, there is no trigger event or reason, but we get home (or wake up) and all we can see is the utter futility of existence and everything positive seems contrived. It's a terrible feeling and a false perception, but it takes a lot of work and often professional (and chemical I.e. Pharmaceutical) help to overcome.
I hope you and your community can support each other and recover, and that together you can all work through the confusion and unwarranted guilt. If you continue to struggle with the emotional fallout, speaking with a professional therapist can do wonders in my experience.
I hope that if there is something after death, that this young man has found peace and comfort - and that the people who held him dear can also find this in life.