I hate point 3 whenever it's used by someone. I've seen it used to defend people who had slaves too. It's like, you know that there were anti-slavery people even back then, right? Heck, there were anti-slavery people back in Roman times.
So if they could see the wrongness of slavery as wrong then, then no one else has that excuse.
It's just that people don't want to admit that a large amount, if not an outright majority, of people in history were bad, evil people. Most people were not good. Or, at the very least, they had little empathy for others outside their immediate family.