The motivation for those Klansmen and NAZIs was clear because they proclaimed it - "America for white people of European ancestry, except for Jews." I assume that anyone marching with them was marching in support of those goals, whether armed or not (though many, if not most, were armed) and whether members of the various white supremacy groups or not. That seems to me a perfectly reasonable assumption. I don't think people who espouse violent racism are in any way "nice" people or "good" people, particularly if they organize under that banner.

It's reasonable to assume that there may have been some FBI operatives marching along. Maybe they were the "nice" people.