Inconsistency?

How is it that the subjugation of the Confederacy in the American Civil War was worth any amount of bloodshed to the liberal left because slavery was 'so evil' yet they were - indeed are - not prepared to extrapolate into more serious areas?
Consider Hernan Cortes and his subjugation of the Aztecs.

Did Aztecs keep slaves? - Yes.
Were they cruel and imperious? - Yes.
Did they use human sacrifice?- Yes!

So Lincoln was a hero for destruction of southern culture; Cortes was a 'bad guy' as he had 'destroyed a local, native culture'.

Do you spot a certain double standard in all of this?