We can argue that slavery actually hurt the South in a couple of ways.
First, it hurt the South because it caused the South to have an economy that was almost completely agrarian and which raised very little in the way of food. The South’s economy was based on raising and exporting cotton. This meant that they very few factories or other industry. This hurt them because it made their economy weaker. It also hurt them when the war started because it meant that they did not have the ability to make guns and the other things that they needed to fight. They relied on imports so much that their ability to fight was greatly reduced.
Second, it hurt the South because so much of its population was enslaved. This was particularly bad for them during the war. The South had a smaller population than the North to begin with and much of the population was made up of slaves. This made it so the South could not have nearly as large of an army as the North.
In these ways, the South was harmed by its decision to keep having slaves.