It's weird we don't have this argument about bandits and cultists.No problem. Humans and orcs can ally for comparable reasons that humans and humans ally. Humans and orcs can fight for comparable reasons that humans and humans can fight.
Just like the Lolth cult is dysfunctional, the Gruumsh cult is dysfunctional.
In any case, the ethical problem arises because the traditional D&D lore about orcs is far too human. To kill one is to kill a person.