I'm pretty sure your average literate roman would have been against his own slavery (being enslaved probably doesn't do wonders for one's auctoritas); the difference was that asymmetric relationships, "laws for thee, not for me", were a little more acceptable then than now.
There is a doctrine in law called the fruit of the poison tree.
That evidence that was obtained illegally (even with a fair degree of removal) is not valid.
I want that as a defendant.
Pernkopf Atlas is a thing, its origins are tainted, we all know it. Yet it's still used. It is very much the fruit of a poison tree. Even if the externalities are awful there is still lots of intrinsic value there. Nothing in the world is free of that taint, and if it is all that one seeks then were going to have to burn everything.
(enthymemes can be effective, but they are utterly reliant on people knowing what the hell your middle term may be)