There's no such thing as objective morality. That's a tired and long-discarded enlightenment idea. Morality was invented entirely by people and is entirely concerned with things people think matter.
We are factory-farming, oil-guzzling monsters that will be judged just as harshly as our forbears for our own barbarities as you judge southerners for theirs. The only reason you don't consider this is a one to one deal is because you were raised in an environment that normalizes and excuses what we do to our planet. The same way enslavers and their communities normalized and excused their behavior.