Those baby-eating aliens produce large net disutility, because the babies hate it. In that case, even without human involvement, it’s a good idea to kill the aliens. To make it comparable, the aliens have to do something that wouldn’t be bad if it didn’t disgust the humans. For example, if they genetically modified themselves so that the babies they eat aren’t sentient, but have the instincts necessary to scream for help.
This situation is more like “they eat babies, but they don’t eat that many, to the extent that it produces net utility given their preferences for continuing to do it.”
Those baby-eating aliens produce large net disutility, because the babies hate it. In that case, even without human involvement, it’s a good idea to kill the aliens. To make it comparable, the aliens have to do something that wouldn’t be bad if it didn’t disgust the humans. For example, if they genetically modified themselves so that the babies they eat aren’t sentient, but have the instincts necessary to scream for help.
This situation is more like “they eat babies, but they don’t eat that many, to the extent that it produces net utility given their preferences for continuing to do it.”