Based on this (obviously German) article it seems like he acted out on some kind of revenge plan against Germans for persecuting and destroying the lives of non Muslim Arabs who try to apply for refugee status in Germany.
Specifically he seems to have fixated on Arab women asylum seekers being harassed by German authorities.
Also there is a good chance he saw himself as white or at least that the skin color played not a big role here. These kinds of ethnic conflicts based on skin color are more common in melting pot societies like on the Americas but here in the old world it is usually a sectarian, political or national. Obviously this is changing though.
For example Breivik killed a bunch of white kids too.