(Was murder, lying and stealing wrong back then? Will they be wrong tomorrow?)
All three of these are listed in the Ten commandments, Thou Shalt Not Steal, Thou Shalt Not Kill and Thou Shalt not bear false witness.
Now show me where in the Bible it says "Thou shalt not have slaves",,, it's like Secession being illegal, you can't prove by man's law or God's that it was immoral or a sin.