msommer I'm not sure of your definition as well because it implies there are no moral absolutes.
I would argue, without any equivocation, that an adult who kidnaps, rapes, and murders a young child is committing an evil act. I can't fathom any possible society or situation in which this would not be an evil act. Can you?
I cannot think of a situation where kidnap, murder and rape would make a large group function better and hence in practical terms all groups reach the same conclusion independently.
Look at certain royal families. Intermarrying has been the norm for political advantage but when you repeat the families too often you get unfortunate side effects and genetically that royal line breeds itself out of existence. This happened in England and they actually had to import a German Prince to take over.
In all of western society (and I believe in the rest of the world as well) marrying your brother, sister daughter or son is seen as flawed and not allowed. Independently we've reached that conclusion - because the effect it had on those who thought it was a great idea.
That doesn't make it a moral absolute but a genetic one.
To my mind it is the same with your example. Over time on the savanna, in small villages we've learned that kidnap, murder and rape have problematic societal side effects. We've made rules about it to avoid having to deal with these side effects on a case by case basis.
There are societies, though, that have placed e.g. familial honor higher than the negative effects of these issues, and that's where you see vendettas where families go after each other for generations, seemingly forgetting the oriiginal slight, but focusing only on the latest score.