I generally don't apply motivation to organisations since abstractions don't have wants; they're just an assemblage of individuals, all acting on their own agendas. Now on the other hand: most individuals that involve themselves in any kind of power structure do so to attain more power; personal, social, economic, whatever. It's just as true whether they're joining the local PTA, the police force, running for public office, or the "let's all wear armbands and beat up/murder people we don't like" club. The motivation as always the same, the only difference is what they choose to involve themselves in to get what they want. It makes no difference if they actually believe in racial purity or are just cynically using it as a lever to move the masses; if the end result is crimes against humanity they're just as complicit either way.