Why did slavery become so awful if God allowed it?
Slavery became awful because man adulterated God's original purpose. Instead of masters remaining benevolent, as God intended, many became bestial and less than merciful. Under early conditions, a benevolent master was viewed as best for illiterate people, but human corruption turned the institution cruel, producing the awful slavery seen later.
Les explains that God permitted slavery in a time when the vast majority were illiterate and incapable of decision making, so a benevolent master was the best situation they could have. In the South, some masters were benevolent and their slaves loved them, having it better than they could have on their own. But man totally adulterated God's purposes: instead of masters being benevolent, they became bestial and less than merciful, and then slavery became awful. As God first intended, slavery would not have been that bad. Les also compares this to marriage: if husband and wife relate as God intended, it should be happy, not a constant battleground.