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>> “The question is why God ordered the destruction of the ancient people of Canaan.” <<
Honestly???
They were pure evil, and their surviving descendents remain so today.
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Both Genesis 3 AND Genesis 6 are pivotal chapters in the Bible. There were nations of Nephilim living in the land, literal descendents of fallen angels. It's no wonder the Israelites were warned about intermarrying with them. Bad seed!
I believe - and I admit to no revelation from God on this matter - that He ordered the killing of the ancient Canaanites because they practiced the one form of sacrifice that is an abhorrent to Him: human sacrifice, specifically the sacrifice of their children to Baal.