Why take that approach?
The simplest refutation of @structureoftruth’s argument is:
Interlocutor 1: “do you think it is possible for (insert moral atrocity here) to be morally permissible?”
Interlocutor 2: “yes. It was morally permissible in (insert historical society here).”
This works for just about any atrocity you can imagine, including rape (of slaves) and genocide (cf the OT).