A political corollary to this that I enjoy came from an old professor of mine- "If your political opinions require large amounts of people to be stupid or evil, you don't understand the issues well enough." (A rough quote, I don't remember exact phrasing.)
The world would be a better place.
Probably these are outliers, so also probably your professor's quote fits well enough most of the time, and only some of the time returns false when it should return true.
"We weren't evil, how can attacking evil people be evil?"
You can see people say things like this even today. The evil in these cases was done by the people convincing the masses that the other group really is evil and needs to be attacked, not by the masses. The masses will attack and punish whatever they think is evil, everyone knows that, doesn't mean that they are evil, rather it is always evil to label large groups of people as evil since it encourages the masses to attack those.