I do not see anything wrong with slavery. You have it in Exodus and in Leviticus. In Exodus it discusses the laws concerning a Hebrew slave. Six years he serves and is let go in the seventh.  In the end of Leviticus, it also goes into the laws of a Hebrew slave that sells himself in order to pay for a a debt. Then it also brings the law of a gentile slave--who must not be let go of. 

In the New Testament also this issue is brought up several times. There the exhortation is for  slaves not to rebel but rather to serve their masters faithfully.  No where is it suggested that a owner of slaves should let them go.

Beside this I might mention that one who does  a favor for someone who does not appreciate it is as he threw a stone at Markulit, an idol that its worship was by throwing stones at it. Few black people feel gratitude towards the USA. Just the opposite. Most are determined to destroy the USA.