My initial thoughts were that we were just absolutely ticked off, showing extreme hatred towards him for all the pain and sorrow he caused us and that was giving us a "dark" aura. He would have known that the fight was more than just us slaying a Primal, this was a very personal fight of vengeance.

After thinking, its hard to say. I do agree that he was thinking he was absolutely in the right, that any who opposed him were evil, so seeing us as this devil-god is a possibility. It could also have something to do with him dying as a Primal, who knows, maybe all the Primals see us like that as they fall (for some reason I recall at least one or two others questioning us as they die); after all, we are 100% against them and do what we can to keep them from bleeding the land dry.

And then there is the possibility that we just plain aren't what we think we are at all (one of my crazy 2.x theories was we were an Ascian who defected to the Mothercrystal;s side. I'm leaning towards us actually being an Eikon now, or maybe even one of the two still-unknown Celestial Dragons or a descendant of one of the 7)