Where were the parents in all of this!?
This world is quite dark as well as bright. In the Black and white of this place there is gray beside each. When you hit one side, there are that of the other and that's just how the world is.
Who knows why the person did it? Who can know? What reason is there for the person to do that at all?
No one for sure, None really.
Depression does screw you up, but that just means there are people who should be there to support them. Even there is a fallacy, because some just don't talk about it.
all I can say is "Damn." and move on. Because it happens near all the time in this crazy world of ours.