Neji is sort of a wild card, because of the history of his clan. MAYBE he thinks he can kill a bunch of people and get away with it, under the fog of war. Or MAYBE he joined Akatsuki, just to exercise free will. But that seems kind of dumb, since he is so weak to his clan (because of the cursed seal).
I guess it's possible the entire Hyuuga clan joined Madara (as a third, but more complicated, explanation).