Honestly, I know all the demons keep referring to Souta as "human" but he's not all a human in the sense of how they use it. Souta is from another dimension. Souta is a "human (a)" and the people of the world are "human (b)." They use the same name but are definitively not the same. Souta is probably as closely related to the humans of this world as he is to the demons.
Frankly, this being presented as a conundrum reflects poorly on Souta's character. The only humans from this world he has met are poachers who are shitty and Nostra who is also shitty. The idea that he would spend all this time with demons, know nothing about the humans of this world, and go "hey i'm gonna betray you for these people i've never met" at all is ludicrous. Obviously he would side with the demons if push comes to shove because he's actually spent time with them and formed friendships.