Yeti's right. You can't rationalize crazy.
For every person who supposedly goes on a rampage like this because of where he's from, how he's raised, his poverty-stricken background, what pills he took, something he learned in school, or whatever attempt is made to put a cause to the effect, there are thousands of other people who endured the same influences and didn't go around killing a bunch of people.
In Alan Moore's Batman comic "The Killing Joke," the Joker tries to explain his own miserable history, and that's why he does what he does.
Batman responds, "Maybe ordinary people don't always crack. Maybe there isn't any need to crawl under a rock with all the other slimey things when trouble hits.
"Maybe it was just you, all the time."