I wouldn't disagree that many slave owners were not cruel. But who among us would want to submit to slavery on the likelihood that we would get a kind owner?
Men weren't meant to be property- not even of a kind owner.
Slavery is and was wrong, I will never say otherwise, but I know that when I read the diary of my ancestor who owned and later freed 50 slaves (before he went to war), He treated his slaves as his own family. One, his personal servant took a bullet for him at Gettysburg, and another was hung by Union soldiers, because he was trying to protect the family property from Sherman's troops. Doesn't sound like these two were mistreated or unhappy. The reality is that most slaves were WORSE off after the war, having to fend for themselves. That doesn't mean I advocate the continuance of that system, but rather that emancipation would have been better if done in a gradual manner.