Well, the prohibition of infanticide is a norm that presumably derives from a perception that killing babies harms the person who does it or others with a relationship to the child (possibly in a very indirect, diffuse way). It is perceived as going against the natural order of things, I guess. Utilitarian ethics does flirt with accepting infanticide in many cases, which is something a lot of people undoubtedly find repugant. Like this story:

"On his faculty page, Mr. Singer argues: “Newborn human babies have no sense of their own existence over time. So killing a newborn baby is never equivalent to killing a person, that is, a being who wants to go on living. That doesn’t mean that it is not almost always a terrible thing to do. It is, but that is because most infants are loved and cherished by their parents, and to kill an infant is usually to do a great wrong to its parents."

https://m.washingtontimes.com/news/2015/jun/16/peter-singer-...