I don't think this is a case of just 'snapping' one day. These incidents seem to just build and build over time. The columbine incident was planned and built up over a long time. They even put explosives in cars so that people evacuating would blow up, but thankfully the bombs didn't go off. They basically were trying to set a body count record. While I've read some of what one of the Columbine kids wrote, and he was obviously psychotic, I think it is also scary that he found a like minded friend willing to go that far.
Back in the 50s or 60s there was a husband and wife who both believed drinking blood would prolong their lives. I just can't fathom how these two hooked up. Was it just one of them who was a nutcase who slowly 'converted' the other to a nutcase or did they meet at the local blood drinking club?
"Come on honey! It tastes great!"
"Well, if you say so... hmm it does taste good! Lets go kidnap teenagers and suck their blood!"
"That's my girl!"
There were a couple of guys in germany a while ago. One of them wanted obsessively to eat someone, the other wanted obsessively to be eaten. How the heck do people like this get together? It doesn't make any sense.
That one was over the internet. A message board posting.