It ended up being about slavery (with the Emancipation Proclamation), and slavery was the root cause for the hatred between the states. They were also very different societies. Almost no immigrants moved to the South, for instance, and there wasn't nearly as much industry.

As another thread points out, if slavery was prohibited in all the new states from the West, it would be game over in Congress, and the slave states knew it. Hence the Kansas-Nebraska Act.

That said: "save the Union" was a much bigger motivator for Union soldiers than "abolish slavery." That's what made them volunteer.

For the Southern soldiers who mostly did not own any slaves, it was more "defending our homes and our way of life" than "states' rights."

In other words, it's more nuanced than you present.