If slaves were at the mercy of their masters why were life insurance policies on slaves common?
Duh.
For the same reason you have insurance on you house. It's value is more than you care to lose. But if you set fire to your house, you wouldn't be able to be able to collect on your policy and you wouldn't be able to collect if on your slave if it could be shown that your care/treatment of him was what led to his death.
You are aware that Southern slaves were cared for when they were young, and when they were old, aren't you? They were paid, and they had Sundays off. Or did Harriet Beacher Stowe leave that out of her advocacy novel? I forget. (It's been a long time since I read it.)
ML/NJ