Good argument: and I've always liked that Lewis quote. It's frustrating when people use moral criticism on people who are actually simply working off different factual preferences.

In terms of rationalisation, I would expect there to be an element of both. It's not very surprising that 19th century Westerners would honestly believe that the Africans they came across were less intelligent: people often mistake people from another culture as lacking intelligence, and that culture being less scientifically and socially (if that can be meaningfuly measured) developed wouldn't help. On the other hand, you have to look down on people when you mistreat them, or else hate them. In fact, I think I remember reading about a psychological experiment where people disliked people simply because they'd victimised them in some constructed game. I'll try to root it out...