Well, demanding that islamic extremists should stop acting like islamic extremists, despite being actually quite a good whish, is the same as asking soldiers not to go to war. You basically have them stop the very thing that defines their role.
Concernig the text snippets: well, it seems he was a religious nutcase after all. But those ain't limited to islam, there are plenty of christians too who think the king james bible is the actual infallible word of god...
But I doubt he was member of an islamic extremist group, or that this thing was planned on the long run. Just an "ordinary" amok run, where the attacker coincidentally was a muslim.
And about religion: it's my firm belief (harharhar) that the world would be a much better place to live in if we would omit that concept alltogether.