Should we not take the southern secessionists at their word when they said that they were acting to protect their slave interests?
The reason Lincoln started the war is muddled. There is evidence Lincoln started the war to preserve the union. There is also evidence that he started the war to end the peculiar institution. Lincoln was like a politician so it's difficult to know how much of what he said to believe and how much to discount.
I don't think Robert E. Lee and like-minded southerners fought for the peculiar institution. Lee freed his s****s before the war.
Acting in the belief that one is protecting one's self interests is a non remarkable position. What were the Northern interests? They were the ones who invaded.