Well, that depends on the situation. If my friend was genuinly remorseful and it was a person I could understand wanting to hate, I´d probably help them out with one murder. Well, maybe not help them, but offer them consolation and other things.
However, if it becomes a regular thing, or my friend seems very unstable, or both, I´d either pull out or call the police, depending on what. It´s one thing risking something for your friend, another to routinely risk your life for them.
Look, if you want to stick with your make-believe Code Of Ethics than that´s just dandy. But if you really think about it, humans have always killed those they perceived as their enemies. In the days of tribal warfare, we slaughtered the other tribes with only the meek excuse of them not being us on our side. That wasn´t some other race, some whole other species. It was us.
We´re animals. Sometimes animals do strange things. Ain´t nothing more to it.