Why is it that history teachers and some schools in general label the pre-Confederate south 'Evil' for slaves?

Modern teachers are judging our ancestors by today’s standards. Standards they were taught by ivory tower liberal professors in college. By the standards of the mid-1800’s, both North and South, those same teachers would not fare any to well either. It is not really fair to judged other cultures or times strictly by the standards you are raised with. There are almost always extenuating cirstances that, if you do not live in the culture or time, you cannot understand or appreciate. I come from a long line of abolitionists and if I had lived during that time I would almost have certainly enlisted with the Union forces to fight against slavery. But as a history buff I understand there is another side to the argument.