If that form of government, that system of social order is not wrong - if those laws of the Southern States, by virtue of which slavery exists there, and is what it is, are not wrong - nothing is wrong.
As soon as war is declared it will be impossible to hold the poets back. Rhyme is still the most effective drum.
Thirdly, even if we assume that the world is governed by purpose, we need only add that this purpose - or, if there are several, at least one of them - is not especially intent on preventing suffering, whether it is indifferent to suffering or actually rejoices in it.