As crude as it sounds, I believe that many past societies throughout the history of human civilization would self-justify the practice of slavery as being essential to survival. Their conditions, no matter which nation or continent, were objectively far more primitive compared to even the most impoverished third-world countries of today.

So they enslaved people from outside of their culture (and, in some cases, from within their own culture) due to rationalizing how it was a "necessary evil" in order to advance humanity as a whole.

Many of them would pepper these rationalizations with religious gymnastics, but it's pretty clear today to a majority of people (theists and nontheists alike) that such "spiritual" justifications were just a smokescreen to excuse authoritarian expedience.

Abridged version: they normalized the sin of slavery in order to cut corners and accumulate personal power through intellectual laziness and physical brute force.