He had an affair with foreign woman resident in Athens; they fell out; he tried to get her sold into slavery. This is presented to the public as seriously shameful conduct—you can’t just sell a foreign woman into slavery, especially when she had been his benefactor. And if presented as shameful

it’s presumably because the speaker expects his audience to agree. I don’t think it’s safe to say they regarded foreigners as sub-human: in this case, it’s clear that trying to enslave a foreigner who had been a benefactor was the sort of conduct that called for censure.

I think respectable Athenian opinion would probably have been scandalised by the American slaveholder class’s selling its concubines and their children.

Just in general: ancients are interesting and important; and sometimes very seriously intelligent and insightful. It’s a mistake to think (worse, hope) they were all proto-Nazis. The scale and nature of enmity between peoples in the wake of European expansion post-Columbus is quite likely new.