My not-quite-5yo daughter:
“You know, in the Lego Movie, the good guys think the bad guys are the bad guys, but the bad guys think the good guys are the bad guys.”
See? Even a five-year-old (not quite) knows that the bad guys do what they do because they think it’s the right thing from their own perspective.
Oh: but if your methods include genocide or pulling limbs or heads off of your fellow man: it’s still evil.
Even in The Lego Movie.
I suppose I could extend this and say if I have not at least thought about the “logic” of why my bad guys are doing what they’re doing – what’s their goal at the very least, regardless of what they’ll do about it – then not only am I not done thinking about it, I’ve not even really started. I’ve just created a series of plot holes with no way to patch them.
It’s probably why the “random stuff in a dungeon all packed cheek to jowl” wears thin faster than one would otherwise think. The plot has to make sense to someone, somehow. And it should be easily articulated.
Perhaps twisted, but easily articulated.