The slave owner has a lower dignity and morality than the slave. Ideas Large plantations need workers. There is no other way the work can get done. The country is being built up. Slaves and others need to be protected from foolishness about states' rights.
Slave labour is highly inefficient.
A system that makes some rich by exploiting others cannot be allowed to exist There are other alternatives to slave labour. Mechanization is providing more efficient and profitable alternatives to slavery. Ideas Mechanization can do the work better. Slave labour is inefficient. Slavery is wrong therefore slaves should be helped.