This is so sad!
I know what it feels like to lose a elementary school-age child only days before Christmas, thankfully I don't know, though, what it is like to lose your child in that violent, senseless manner...
Somehow I just cannot believe that anyone can do something as terrible "just like that", or because he/she played too many cruel video games, or watched too many cruel films on TV, or had access to a weapon.
Things in that person's life must have gone frightfully wrong for a long, long time. A horrible act such as the one that just happened has a story. There is a prolonged period of build-up involved before it "explodes" in this terrible, frightful, totally unjust manner.
And no kind of education, deprivation of access to fire arms, preparation on the side of the victims (who can you prepare for anything as insane as this?) - nothing could have stopped what took so long in its coming. I don't know where in the life of this 20-year-old things went wrong, and I am NOT trying to excuse neither this individual, nor his deed. But I still believe that no one is simply born this bad - something made him lose his mind to this horrible extent.
Probably - if anyone had looked closely, paid enough attention - whatever did go wrong in the attacker's life could have been seen. But it was probably considered as "family business", "nothing I want to get involved in", or "...not too bad".
That's my guessing - what do I know?
I can't even fathom what my niece feels like inside who's severely afflicted by depression and schizophrenia (for example)...