“If the South had freed its slaves why would it have seceded in the first place?”
For the same reason it did in reality: To retain its economic interests. The South believed that the United States, as created and accepted, was a nation of individual sovereign states that entered into a compact for certain mutual benefits, and through that compact the states allowed for a small and limited federal government to have certain enumerated (specified) and limited powers, all other powers to remain with the states and the people. Abe Lincoln (and some of the Northern states, but not all) thought that the states were subordinate to and subservient to the federal government.
What economic interests would that be?
To retain its economic interests. The South believed that the United States, as created and accepted, was a nation of individual sovereign states that entered into a compact for certain mutual benefits, and through that compact the states allowed for a small and limited federal government to have certain enumerated (specified) and limited powers, all other powers to remain with the states and the people. Abe Lincoln (and some of the Northern states, but not all) thought that the states were subordinate to and subservient to the federal government.
And what was it exactly that the mean old Republicans were advocating that suddenly upset the applecart?