Treatment of slaves varied widely, depending on a variety of factors such as number of slaves (hence their individual value to the household), type of work the slave was assigned to (a field hand was more likely to be brutally treated than a house slave whose job was to care for the mistress' personal needs), the master/mistress' individual natures, and really any factor that could vary in the situation, which were many. Some slaves were treated very kindly by their masters. Others were brutalized and even murdered. It is worth noting that even slaves who were themselves fortunate enough to have a kind master often mentioned in interviews that they knew or were aware of slaves who had been treated horribly, and that slaves who had been treated horribly seldom included mention in interviews of knowing or hearing of slaves who were treated kindly. Cruelty, even extreme cruelty, to slaves does seem to have been fairly common, especially in the large plantations.

I've found two online collections of former-slave narratives/interviews, which you can peruse at your leisure as primary sources are really best when it comes to this kind of thing. The first is a lot smaller than the second, but both are valuable resources. A note on the first: The interviews were recorded in dialect, so they might be difficult to read if you aren't familiar with how a strong Southern accent might look as written phonetically.