If I recall correctly, doesn't Diane Duane explain the difference in Dark Mirror
as being somehow an effect of a subtle difference in the universe's make up?
But truth told I think that's a cop-out excuse. Humans are capable of incredible and shocking cruelty and be perfectly sane. Raise them with different standards of what's acceptable and what isn't and see what happens - just look at Germany, falling under the spell of Adolf Hitler and the atrocities that were committed.