Hopefully the MC makes an entrance declaration like "I am the Harbringer of Pestilence, the Emissary of Death. I have come to reinstate the balance of life and death. For in this place new 'life' has sprung forth in an irregular way threatening that balance." while looking at the ogre army.
Then march towards that demon and kill all ogres on the way with his stun + poison combo.
If a human approaches him he should simply also kill that human in that way.
When a human (most likely the hero girl) then asks why he killed the human, he should reply "I do not care about humans, demons and the strife between them. I only care about reinstating the balance between life and death. As long as I ensure the balance to be obtained again, it does not matter who dies in the progress - be it demons or humans."
That way he would establish a third faction and probably avoid anyone from realizing his identity.