When you’re hungry you gotta eat.
If you think about it, ancient humans I am sure observed animal behavior. Animals will kill and eat even their own. Ancient humans were not much more than animals themselves so killing one of their own and eating it wasn’t much different than what they observed in the animal kingdom. Apes and monkey do it, birds and fish do as well. Male lions will kill young and eat them. Coyotes will kill and eat dogs. Sows will sometimes give birth to a litter and eat all of the young. Killing and eating has been going on since the beginning of time, I am sure there was no one there that told them they couldn’t or that it was wrong. No it is not surprising at all.