Of course it was about protecting the institution of slavery!  "States' rights" concerned the right to keep humans in perpetual bondage, to buy and sell them, to torture and rape them.

Not framed in those terms.  Southern politicians claimed that holding blacks as slaves created conditions for all members of their society, including the slaves, superior to the inequities and strife of northern and European society.  But the large body of slave narratives shows this lie for what it was.  See, for instance, Frederick Douglass' autobiography, or Solomon Northrup's TWELVE YEARS A SLAVE, lately adapted into a movie.

If you doubt southern motives as they existed before the Civil War, please have a look at speeches by leaders of  the southern states before their secession.  You can find some of those speeches at http://www.confederatepastpresent.org/index.php?option=com_content&view=category&layout=blog&id=41&Itemid=54&limitstart=10 .  "States' rights" is post-war whitewashing.