I think it's fairly obvious what the motivation would be for some group or person to try and invalidate the concepts of good and evil in other people's minds.

When your victims can say nothing but "who am I to decide what's right and wrong?" then you can do whatever you like without fear.

Just as an aside, a similar canard is the concept constantly pushed by the Left that the only valid decisions and judgments are those made by a collective.

I am constantly amazed by the number of people who condemn all our actions in Iraq who would have applauded the exact same actions had they been "blessed" by the United Nations.