Any reasonably intelligent person who's actually read the Bible cover to cover a few times would not walk away with the Bible justifying slavery. It simply does not do so. The "slaves" of the Bible were more like indentured servants as they had many rights compared to US, slaves who were treated as chattel worse than animals often times, and had no rights to redress grievances. U.S slaves could be killed without impunity. The Bible actually proposes rules of protection for servants, or what you consider slaves. The Bible also says that all servants should go free during the Year of Jubilee. There are many verses in the Bible that provide rights for slaves. If you read the books of Leviticus and Deuteronomy you would see that.
Deuteronomy 23:15
“Thou shalt not deliver unto his master the servant which is escaped from his master unto thee:”
Exodus 21:27 And if he smite out his manservant's tooth, or his maidservant's tooth; he shall let him go free for his tooth's sake.
Deuteronomy 24:14 Thou shalt not oppress an hired servant that is poor and needy, whether he be of thy brethren, or of thy strangers that are in thy land within thy gates: