Pelham, you are correct that slaves were held in all the states prior to the Civil War. However, the holding of slaves by the South was different in both quality and quantity. Slave breeding, the breaking up of families, and harsh and brutal treatment eclipsed anything in the North. But the sin was the US’s as a whole and God judged it in whole.
As for Jewish slavery in the Biblical times, there again is a massive difference in quality and quantity. it can be easily shown Biblically and historically that Jewish slave treatment was better than anything the South did to its slaves. And God expected the Israelites to treat them better because of the effect on non-believing nations.
Of course the old Presbyterian divine Robert Lewis Dabney appears to disagree with you.
http://www.amazon.com/Defense-Virginia-South-R-Dabney/dp/0873779290