> I'm not aware of any instances where Jewish people were rounded up purely for being Jewish. There was always some misguided and wrong justification, like a belief that the plague was caused by Jewish people poisoning wells

At that point though, those Jewish people were in fact persecuted and murdered purely because of their ethnicity. It matters not that the persecution was justified with stories of poisoned wells. What matters is that those stories pertained directly to a specific ethnical group, and that group was subsequently hunted.