actually slavery was one of 2 direct causes,the other tarrifs ,both economic reasons.the states rights issues came about as the southern states rejected tariffs and were reliant on slavery economically,and paid around 90% of all us taxes back then,through tariffs.the tariffs also crippled trade between the south and its trading partners,and severely crippled its economy.
now add in slavery,taking that away from the southernors would have made the south then like detroit now.add in the fact none of these things harmed the north in any way but the north recieved all the benefits,while the south paid all the cost.
and even something few people think of,the north wanted cotton prices in the south to rise,while putting tariffs on it to keep countries like britain from getting cheap cotton,so northern textile mills could corner the market on clothing,instead o the south selling cheap cotton to countries like england who were direct competitors to northern textile factories.