Maybe on the successful Earths, mass genocide is acceptable in the case that the country involved is an obvious failure, such as north Korea.

This is a confusion of terms: "acceptable" is a cost-benefit calculation, and the cost of genocide is determined by human nature, by how much we value lives, independent among the Earths in this construction. If a certain variant of Earth considers genocide acceptable, it points to an epic failure of rationality (possibly due to something along the lines of scope insensitivity), and so can't be "successful".