David Weber places a lot of emphasis on this too; I wrote down what I could remember of his discussion of the topic at ICON 2012:

Then Weber went onto a tangent I really appreciated: while working 4 assistantships at a university, he would tell his class that Hitler's actions were all highly rational & understandable if one understood his world view. An important writing rule: have no simplistic villains. The villains must have good reasons for everything they do.

Weber gave an example: the Mesan genetic slavers in his Honor novels. They are breeding a master race, and during the centuries, they have blighted the lives of billions - but they are all still human. So he described a scene from a book:

The leader and his wife are preparing for dinner in their rooms. The wife - "Oh honey, don't wear that red shirt." The husband: "but that's my favorite shirt!" Wife: "I know, and hopefully the geneticists can do something about your taste. And you're not wearing the red shirt."

(Everyone laughed).

A good writer makes bad guys comprehensible; hence, some fans come to opposite conclusions about Weber's politics, based sometimes, he said, on the same exact passages from his novels.