11 out of 15 victims were female (8 pupils, 3 teachers). Sadly, we don't have any Amoklauf note (being able to make up words on the spot does it) and thus don't know exactly his standing on society.
My guess is, as I know of a pretty good source (my brother, who worked for the shooter's father for a while), that he just got turned down by one of 'em and, thanks to his depression, made it into a Damn Important Event, culminating in the shooting. As for our Killerspiele (killer games, violent games, whatever), pretty much every male teenager got at least one of them; if they did make normal people into monstrous, asocial, merciless killers, then we'd have a lot more oh-damn-that's-terrible weeks of grief.