A very important question becomes who decides who is evil and who is good. It might be clear-cut for the demon but what about the town blacksmith whose been accused of a grizzly murder? Are you going to torture him to get a confession? Everyone in the town now views him as evil, why wouldn't you?
Another issue with selective moralism is the inherent hipocrsy of it. If demons become special entities that you can commit any depraved act upon without it being evil, it follows that one could seek them out simply for the ability to induldge in heinous behavior with impunity.
For instance, a wizard summons an attractive female demon and imprisons it for the purpose of fulfilling his lust. The demon does not enjoy it's captivity and is routinely sexually assaulted by the wizard. Is this behavior condemnable, or does the fact that it's a demon change things?