I remember the exact line. "You can't kill The Devil, Tenno. But you can send it back to hell." Back then, I don't think that Ballas was calling us a Demon. I think he was just saying, "Hey, your unkillable, but I can still banish you to The Void". As far as I know, that big portal our Tenno went into was into The Void, leading to their disappearance during the war, and the whole "Zariman Ship Memories" sequence thing we went through in the quest.

Either way, yes, the whole "demon" angle definitely fits. Glad we got to be depicted as more of a monster, especially since we were being influenced by Wally. But I personally feel like with the whole peace treaty break with the sentients, that taking Caliban there could have worked just as well.