Teenage angst, hormones, probably a bad personal situation... girl left him, daddyless, broke, drunk mother, a rough poverty striken life... so either, or, or any other bad scenario of the sort could have played a part. I saw that disturbing vid someone posted here, he was obviously broadcasting, looking for approval from somewhere, be a hero.. basically a guy who had given up on life..
most terrorists have a similar story, racial or religious... circumstances drive them to insanity and into murderous rages.. very sad, the needless loss of life.. RIP