What makes you so sure? Slavery was universal across the human race, or practically so, until well into the second millennium AD. Nobody seriously questioned it. A bit like interest-taking now, which is why I brought it up as an example. Borrowers may complain about their monthly payments, but they don't seriously imagine themselves the victims of injustice qua borrowers.
We imagine that "if I lived back then, I'd be against it", but we forget that the past is a foreign country. People thought and acted quite differently.
Just from a point of common sense logic I would have to assume the vast majority of the slaves' arguments on the subject.
Let me just pull up all of the surviving primary sources written by the slaves of the that period... Oh, right.