What if they both want to help humanity? Angels do it by showing humanity their good sides - demons do it by showing humanity their bad sides. Both are trying to push humanity to transcend itself, in different ways.
What if they're actually reversed? Angels try to damn humanity by promising shortcuts and simplicity. Demons try to help humanity through purification and inner truths.
I can see there's some interesting stories that could come out of this.
That's all. Back to your regular programming.