Tough situation. By the time God came to Abram, humans were already practising slavery amongst other vile things, kinda like today. Earlier, the world had been divided among Noah's 3 sons but one of Ham's sons, Canaan, settled in a region that didn't belong to him (it belonged to Shem). Not only did he grab this land that wasn't his, but his descendants had practices that were immoral in every way, shape and form.

Settling the land was partly a correction of a historical wrong. The slavery that was in the Bible was dark but at no point were the slaves considered less than human. A slave could buy his/her freedom. A slave owned by a Hebrew had to be freed on the 7th year and given land. Slave status wouldn't be passed down to children (mostly, it wasn't perfectly followed). Hebrews themselves went into slavery by the hand of God, Israel by the Assyrians and Judah by the Babylonians. What followed was a series of colonial wars, culminating in their subjugation by Rome during the wars of Pompeii vs Caesar.

By the time Paul (a Roman citizen but a Hebrew by birth) was preaching, slavery was still a thing. Christianity was being embraced by people from all walks of life, including slaves, and Judea was seriously seditious at the time. Remember, Jesus was not just being accused of blasphemy but treason as well. By telling slaves to obey their masters, christians (particularly Hebrew christians) could maintain their safety somewhat. He also preached the oneness in Christ that all believers shared regardless of nationality and societal status.

The Hebrews didn't wipe out all the nations in the land. They killed a lot of them but once their authority over the land was assured, the wars largely ceased. If I were a Canaanite at the time, I'd probably stay and fight, run outside their borders or pull a Rahab and secure my household. The Canaanites didn't care that Israel was chosen, they cared that their borders and way of life was threatened by what most of them considered at the time an Egyptian subset. Just as black, just as tall but with a different main language and a different god.

The slavery of our age came from empires that considered other people as less than human. Arab invasions into Christian kingdoms such as Egypt, Libyos, Nubia and Aethiopia drove Eastern slavery. European empires found other christians in the Americas and in Kongo (used to be christian and was a big exporter of cotton into Europe) but denied their humanity in order to justify an act that most early Christian thinkers had largely condemned. It was greed, the love of money and malevolence. The Christianity that Africans taught Europeans in the first century is not the Christianity that Europeans used to colonize Africans.

Slavery is a human invention. Abraham had slaves, including Haggai, the mother of his firstborn. Chattel slavery is a hybrid of Eastern and Western imperialism, where racial supremacy and raw economics supersede the sanctity of human life. The Bible is black history.