Since the North was more industrialized and more of the economy was based on manufacturing, so the society didn't believe that this was a positive economic contribution for America. It wasn't wrong to them, but they just thought that the Slavery was unnecessary. Slaves who were free, also known as free blacks, had way more rights than slaves in the South. They could be individual farmers or craftsmen. Although, they were segregated and didn't really have that much freedom as much as most of the society. Some free blacks wrote poems or stories on slavery and their point of view. Some of these men Harriet Douglass and Harriet Jacobs. Not many free blacks who were women didn't have as much rights as black men.
Slavery in the South:
The South had an economy that depended on agriculture because of the mild climate, rolling plains, and rich and fertile soil. Large plantation systems had to hire a lot of farmers who need a pay, so slaves were a huge help, and later on a necessity. If a farmer with a plantation doesn't have any slaves, the business will not survive. They wanted to encourage the North to keep the act of slavery. There were barely any free blacks that live in the South.
Slavery in the West:
There was a lot of pressure from the North and the South to support slavery or anti-slavery. In 1846, during the Mexican War, Representative David Wilmot of Pennsylvania was going to limit slavery in the West through an amendment to a war appropriations bill. His proviso, banning slavery in any territories acquired from Mexico, passed the House but not the Senate. The Wilmot Proviso--included in subsequent House bills but never enacted. This grew a great debate among the West.