"is there anything ethical about, say, slavery?"
KK> No there isn't.
According to the group selection theory a group of individuals or a
population that uses violence to solve conflicts is at a
disadvantage. A population that uses threats or that is
non-aggressive has the advantage.
It is possible that a surviving aggressive individual could meld into
a non-aggressive group. It is also possible that a member of a
non-aggressive population may decide not to abide by the
advantageous non-aggressor rules. Over time a non-aggressive
population could lose its shared advantage.
Entropy?
Kevin