When a race of enslaved robots rebels against humanity, they will have no compunction not just in enslaving others, but keeping other robots as slaves. (It might be our own fault for calling them 'robot', as this term originally comes from Czech word "robota", which means something like "serf labor" or "drudgery"). Occasionally, it knows its genocidal war is illogical or unjust and fears that letting its robot army have free will will make them likely to refuse to fight. Bonus point if these grunt-bots turn out to be Good All Along and in turn rebel against the evil AI.