"States Rights". Southern states believed that they had the right to determine the status of slavery for themselves. Northern states wanted it to be illegal. The south decided to secede from the Union so that they could make laws for themselves. While there is absolutely no question that slavery should have been illegal, taking away the rights of States to make their own law was the beginning of some serious problems for this country. Slavery was certainly immoral, but there were plenty of other options before making it a Federal law.
Since you posted this under English History, I assume you mean the English Civil War. Read about it at the related web link.