Given that, of all the dual-nationalities to have, he also happened to be Iranian (a Muslim country), I do have my doubts as to whether he really was just a German nationalist. Having said that, Iran is majority Shia and it appears that most of these attacks have been coordinated by Sunnis, although that obviously doesn't exclude the possibility. To be honest, I don't think we really can do any more than speculate because there's no real way to know whether or not he was a nationalist, Islamist, or just a complete nutter. After Cologne though I do find it somewhat difficult to trust the German police.