The point was that it does not take overwhelming planning , if two people with mental issues (your word) can do it.
The hard part is getting 3 people willing to die so a certain number of random other people they probably never even met with will also die, whereas the organizer will live - and the perpetrators know all of this.
Organizing the attack itself it outright trivial compared to that.
Incidentally, this excludes Columbine as a valid comparison on multiple levels, because they targeted people they knew (and had plenty of conflict with, as far as I know), and didn't leave any member of their club behind alive to benefit from their deeds.
McHrozni