Yes people sometimes reject what the Holy Spirit is telling them. But you don't need the Holy Spirit to tell you that slavery is wrong unless you don't have a conscience. And if you don't have a conscience then you won't listen to the Holy Spirit anyway. I think that the Holy Spirit is more concerned with spiritual matters. 1 John 5:19 says that the whole world lies in the power of the evil one. This was written after Jesus was crucified. I don't see anything in the Bible that says that it's different today. It isn't, and it won't be until the second advent. The fact that slavery is illegal now isn't a spiritual matter.