I really don't think that the Atsuki is going to seal the demons into themselves it really seems like thier using the demons to unseal/unlock whatever that super demon thingy is supposed to be. The only reason why they refer to getting their own human sacrifice is probably because they've all been assigned different demons to track down, not for themselves but to bring back so they can be added to the super demon thingy. We still don't really know what the hell their ultimate goal is so all we can do is speculate at what there exactly doing with the demons.