Old Walther's views on slavery are available for reading in a translation by Erika Bullmann Flores from the old Missouri Synod publication Lehre und Wehre. http://www.reclaimingwalther.org/articles/cfw00002.htm
Old Walther was worried about humanism. He defined humanism "as the belief in a human ideal, a belief that man within himself has the ability to develop into a state of completeness and achieve happiness."
Walther didn't want to talk about slavery "as it relates to political issues, for we have nothing to do with that, but as it related to Christian-religious morals."
Walther said that emancipation is a government issue, but it is not a theological issue. The issue is "whether slavery itself, that is, the relationship between slave and master, is a sin; or does sin adhere to this relationship merely in concreto, as all relationships between sinful men, for instance between poor and rich, seller and buyer. Is therefore slavery a sin which must be unconditionally opposed, or should Christians concentrate on doing away with the connected sinfulness, so that the relationship between slave and master is according to God's will and order, according to the laws of justice, fairness and love?"
And suddenly Walther shows his hand. "We therefore hold that abolitionism, which deems slavery a sin and therefore considers every slave holder a criminal and strives for its eradication, is the result of unbelief in its development of nationalism, deistic philanthropy, pantheism, materialism, and atheism. It is a brother of modern socialism, Jacobinism and communism. Together with the emancipation of women it is the rehabilitation of the flesh."
And old Walther still isn't done. He continues: "As proof of this blood-relationship it suffices to point only to its history, but also to the close union between abolition-minded representatives of Christianity and the abolitionist tendencies of anti-Christians and the radical revolutionaries in church, state, and home."
Well, there's more, but I think that old Walther has made his point. He proceeds to consider the definition of slavery. Proceed to #4 in this series.