There are several possible arguments, some more outrageous than others.
First, in the U.S., the methods used to end slavery violated states rights and local autonomy. You could insist that what was done to end slavery, and the deaths in the Civil War, harmed people far more than slavery itself.
Next, there is an economic argument. Because slaves are property, it is in their masters' self interest to keep them alive and healthy. If a slave dies, the master loses money. When the slaves were freed, it no longer mattered if wages were so low that they starved to death. An employee is merely an expense (whereas a slave is an asset).
How can someone legitimately claim that enslaving humans is wrong while raising billions of animals for food is right? Slavery hasn't ended; we have mere decided to inflict it only on species too weak to defend themselves.
Aristotle argues that some people are "natural slaves," especially the pale races of northern barbarians who have naturally sluggish dispositions due to the cold climates in which they live. Rather than end slavery, why not just pass laws ensuring the well-being of slaves?