15. It has all the psychological benefits of a giant murder-suicide.
The desperate person isn't content with just ending his/her own life (usually a he, let's be honest here), he wants to take people with him. It's like thinking, "I'm going, and I'm taking all you lousy mofos with me!" without having to actually do anything. Nobody has to be the bad guy. Nice people can join the fun.
I am convinced apocaypse ideation is wishful thinking about 95% of the time.