This is not meant to defend cruel people, I just want to raise a few points about slave owners.
While I have only been to small plantations, I have definitely seen what some people would consider slave cruelty. But I think it's important to understand the mentality of people that work with these slaves.
Slaves on plantations are seen as property, not an employee in any way. Most owners and workers of slaves distance themselves from the slaves to keep themselves mentally healthy. When distance yourself from a a slave, you won't be treating it as nicely as you would your hired hand.
Plantation owners see slaves as money, so they won't do anything that could jeopardize being paid (won't damage the product).
When people are disconnected with the slaves they are working with, it is easier for some people not to be so nice to those slaves. This isn't to say that all people working with slaves will be abusive towards them, but it creates an opening for those people that aren't as nice to take their anger out on the slaves.