That is, murdering someone is so socially inexcusable that we have evolved to instinctively avoid murdering people - or doing anything that is close enough to count as murder in the eyes of our tribe.

Is evolution fast enough to have evolved this instinct in the past 4000 years? IIRC, anthropologists have found murder was the most common cause of death for men in some primitive tribes. There can't have been a strong instinct against murder in tribal days, because people did it frequently.