Most of the early "important" killings (Ben, Marty, Thomas and presumably Hunter) seem certainly like they were done with motive and in Lucy's case it was presumably just because they were going too close to the woods where we has hiding (which is possible for Booth too, I guess?)
I'd have hated for the writers to try to have him justify why he murdered someone like Nikki - I can't think of any famous murder-mystery with 15+ victims all carefully explained by the narrative.















