Phage

Ok, lets look way back into antiquity where we have Village A, B, and C. Villages A and B hold moral views that are pretty close together, but have enough differences that they might give each other funny looks sometimes. Unfortunately Village C is also quite similar to A and B except with the distinction that it is considered perfectly OK to steal and eat someone's baby. In fact there is a festival every year devoted to the sport.

Obviously Village A and B are going to get rid of C as soon as they can. My point is that the societies we have today are subject to the societies around them, and there is a certain level of difference that will result in them either changing or being eradicated. It would be expected that such basic moral values would equalize over time and exposure to others.