Although my experience with this is severely lessoned in comparison to a school shooting, we did have similar problems in my family with my older brother. My mother (his step mother; they're not related whatsoever (I'm his half brother)) would purposely sleep with her door open and all of our doors shut when I was younger, because she feared he was going to do something to one of us. She had no "proof" necessarily that pointed to his being evil incarnate, but she felt something by looking into his eyes. My dad was the type that always denied anyone doing things simply because they enjoyed torturing other human beings, and whenever she would bring anything up he would brush it off. Well, she was right......completely right, and she wasn't there when shit actually hit the fan. Things are okay now, but a lot of things happened that needn't have occurred.

I think it was looking back at previous events through a new lens prescription, that makes things easily identifiable clearer; the difference between recognizing something is a tree and seeing each individual leaf and gnarl in the bark.