NOW we regard slavery as a terrible thing, as it should be. But it wasn’t like that at the time. What is ethically fair to us is not always fair. To give you a pretty stupid but stylish example... we are now in the age when we use technology without any problems, and if a PC or a cell phone breaks, we throw it away to buy another.
It’s a very normal thing, who would be outraged about a cell phone thrown away?
Here. Let us say that in two centuries the technology will be so developed that it will be possible to create AIs so refined that they will have a definable human consciousness and with rights equal to ours.
So, we’ll be horrible monsters in the eyes of those people of the future, when they think about how we exploited and disposed of our machines without a penny of pity considering them rubbish.