What did people mean by slavery being a "necessary evil"

The southerners argued that slavery was a "positive good". They argued that the slaves were in actuality happy, content and well cared for. They even went as far as saying that being a slave was better than being a worker in a northern factory, a condition referred to as "wage slavery."

Before 1830 - The South defended slavery as a necessary evil. They argued that the emergence of cotton as the most important cash crop in the country made slaves necessary. Instead of defending slavery as a necessary evil, they began to defend slavery as a "positive good."