If he was a neo-Nazi, the odds are very poor that he was mad at moslems. Most neo-Nazis support the moslems 100%. Why the 9/11 tattoo then? Either to celebrate or because he believed "the Jews did it."
Who are the chief victims of neo-Nazi violence in German, pray tell?
I assume you meant "in Germany."
If the group/individual is actually neo-Nazi, then its target is always Jews. German Nazis have always been pro-Arab and other "far right" European organizations (such as France's National Front) are also pro-Arab/islam. Just as with David Duke in Tehran, real Nazis will always make common cause with the enemies of Jews no matter who they are.
If you're referring to boorish nativists, every country on the face of the earth has people like that. But if they're Nazis and not just common ordinary nativist thugs they will support the Arab/moslem world against Israel and Jews.
Also, one would expect any real Nazi worth his salt would recognize Punjabi-speaking Sikhs as his fellow "aryans."
If by "neo-Nazi" you mean the last remaining Nazis it could be that Jews are still the main target, but if you mean younger people who've gone in for racial extremism, that may not be the case any more. Just as Nazism was something different from governments and political movements that came before it, successor movements in that line aren't going to hold exactly and without variations to the ideas of the 1930s and 1940s.
And while some Muslims and some neo-Nazis or post-Nazi racial extremists may consider that they share a common enemy, murders in Germany and talk about "mud people" over here indicate that there isn't "100% support" either way. For the record, I don't think the killer thought he was attacking Muslims, though. I don't think he was thinking clearly at all.