I'm pretty sure that will be one of the outcomes.
Personally I can't really think of much that can be done to actually prevent someone doing such a thing in a protected cockpit environment if they have such a motivation to do it.
As for depression, well plenty of people suffer from that in one way or another, but it's a very long and convoluted pathway from depression to mass-murder.
There are 'crazies' on almost every corner. Was it always like this ?