The "freedom to enslave" is one of the most immediately obvious contradictions in the philosophy of the Confederacy during the Civil War. This contradiction has been covered up with a number of convenient arguments, most notably the assertion that the war was a matter of "states' rights." Of course, the obvious implication in that argument was that states had the right to enslave human beings if the government of that particular state decided that it was permissible. There were no other tangible or pressing matters of "states' rights" that were being oppressed, certainly none that merited a war.
In terms of personal philosophy, the answer is as obvious as it is horrifying. The only way that someone could internally rationalize a war for freedom while simultaneously fighting for the right to enslave other human beings is if they were incapable of or unwilling to see the enslaved party as meriting the full rights of a human being. Indeed, this seemed to be central to all of the rationalizations of several Confederates. At best, they assumed that the imported Africans were a subservient type of human, and were best suited to the sort of manual labor that they had designated for them. At worst, they considered them inhuman, and more akin to beasts of burden. This mindset and subjugation remains one of the darkest chapters in the course of human events. If there ever was a just cause for war, it was the plight of the Union to free the enslaved. However, historical accounts indicate that the Union started the war to preserve the United States rather than to emancipate the slaves, and would later pivot to that cause.