On twitter a lot of people are essentially saying "Good, now we'll get answers." I'm sorry to say I don't believe there is typically anything satisfying about the answers of people who become deranged. Maybe this time, but if reading about madmen past is any guide, its very unlikely that we will be enlightened by any testimony.
Answer is really simple: in his own world /subculture he is a hero, a martyr and his name is being mentioned worldwide.
Or maybe it's the OTHER simple and cultural answer: they were seen as "losers", then lost it and decided to kill random people. Pretty much like Columbine and all the other "seemingly normal kid kills random people with guns" incidents that keep happening, again and again. It doesn't matter, though - at this point, it's all just random speculation.
Answer is really simple: in his own world /subculture he is a hero, a martyr and his name is being mentioned worldwide.