I wonder if that dark witch
didn't decide on becoming a demon? Maybe those horns and her discoloured skin are actually signs of a curse or a brain parasite that's controlling her, rather than any allegiance she decided to join?
She did grab her head and start screaming when she thought about it.
It seems to me that the dark king has mind control magic and is compelling the forces of darkness to attack, because it's basically when he appeared that she started acting weirdly.
Prior to becoming a demon, she had clearly started worshipping the dark god of her own free will, and even as a demon, talking to Sekai, she is telling him of her values of balance between light and darkness, so I don't think she's being controlled (at least, not when the dark king isn't around).
The dark king is intent on invasion, and Veradonna also appears to have some extremist negative views she is spreading regarding the darkness. We'll have to see how this situation came to me.
But there might indeed be some sort of curse, I think it's strange that all the darklings look the same.