I don't know where people are getting "explanation" = "excuse," either. Then again, I haven't read the whole story line.
Fleshing out a villain's background is a call for reason, I think. It's the idea that nothing a person does, however heinous and unforgivable, happens "just because."
Some readers like to see that, and some don't, I guess. I probably tend toward liking it because I was raised by a mystery fanatic, and the whole point of most old-school mysteries is to impose order on an otherwise chaotic-looking universe.