Was slavery was justifiable in the south?

It depends on your outlook on the free human labor that the newborn nation was able to utilize, it certainly benefited the economy for America, and I feel that is what many southerners felt justified the need for slavery. Of course, however, there were the members of the Ku Klux Klan (KKK) or Solid South which would simply have justified the fact that Blacks (they weren't really considered African Americans just yet since they weren't necessarily born in the U.S.) were inferior.