Also note that slaves were treated very much differently in different times and societies - most people's image of slavery is that practiced in the deep south in the early 1800's, when there were many different methods and means - just for example:
The African slave trade developed because when tribes made war, they took slaves... who would serve for a time, and then be freed; their children were not slaves, and they were still people (even though owned). A bad thing, but could be recovered from. Pity the slave-traders who purchased them didn't follow the same rule-set....
There is biblical support for slavery.... which had a definate term (at least, for some groups....) a male hebrew slave was to be given the option of walking away after a set period of time (or was it at a set date, regardless of duration? I forget....); likewise, a slave in your service who was severly injured (lost an eye, hand, or foot) was to be released a free man, and a master who beat a slave with a rod, and the slave died of the wounds that day was to be punished (but I don't recall a listing as to what the punishment was specifically to be, who was to administer, and the like), and the like.
Not all slavery is equal.