Eliezer, if you had been raised a Yanomamo you would not consider murder to be so horrible.

I believe the a sizable portion of Yanomamo killings are revenge killings, which indicates that Yanomamo do think murder is something really horrible. Otherwise, why risk life and limb to avenge/deter it?

Chagnon also stated that occasionally Yanomamo tribal leaders took measures to prevent warfare, such as deliberately underproducing and overproducing certain goods in their village in order to give them an excuse to trade with each other, making it easier to cement alliances. He also recounted that more cool-headed, logical members of the tribe (Kaobawa was one he talked about a lot) seemed to better understand the inherent problems of killing and took action to prevent war. This indicates that Yanomamo are probably psychologically normal in their response to violence, and the reason they are so violent is due to local conditions, not because (as I think you imply) humans are blank slates who only dislike killing if they're taught to.

Also, there is considerable controversy over whether the portrayal of the Yanomamo as so incredibly violent is accurate, although I did not have time to figure out which side of the debate was correct.

If people were genuinely taught to believe that God hates using the bathroom at that time as an abominable sin, we would fear it.

No one is disputing that, the question is: Would we keep fearing it if we then learned there was no god? We probably would still exhibit fear behaviors due to cached thoughts, but it seems likely to me that our conscious minds would know that those fears were irrational.