That is incorrect.
Slavery was still a protected practise under the Constitution of the United States and had been reaffirmed by several United States Supreme Court decisions not the least of which was Dred Scott. It was a practise which was even followed by Northern states as well and had to be eliminated by the passage of the 13th Amendment to the Constitution AFTER the war was over. 8 of the 27 state which ratifiedd this amendment were OCCUPATION Government in southern states which ratified the amendment BEFORE the war had ended. Those 8 states along with the 5 states which rejected the 13th Amendment in 1865 would have been enough to have kept the 13th Amendment from recieving the 3/4 majority of the 36 states then in the United States needed to amend the Constitution. In other words the passage of the 13th Amendment, which ended slavery, was, even in 1865, procedurally highly suspect.
To preserve slavery all the southern states had to do was REMAIN in the Union and vote down any amendments to change the practise. No amendment to the Constitution could have been passed eliminating slavery as long as 1/3 of the states voted against such an amendment. In 1860 1/3 of the 33 states within the Union seceeded. 4 more slave States, Missouri, Kentucky, Maryland and Delaware remained in the Union, althought Missouri and Kentucky were split, and Maryland and Delaware were forced to do so at the point of a bayonet. 2 other states Illinois, especially southern Illinois, and New Jersey were also leaning proslavery on the idea of freeing the slaves.
The abolishionist were those people who were in actual rebellion against the United States and its Constitution, such as John Brown at Harpers Ferry. The Southern states were abiding within their rights given by the Constitution, the laws of the land, and the rules for governing this country at that time.
Morality does not equate to legality, unless you wish to live in and be ruled by a Theocracy. We were then a Republic.