User blog comment:B214/Question and Answers 6/@comment-120.20.138.121-20170109132412/@comment-29963364-20170112025929

Yeah. But those Angels just leaked information to the Khaos Brigade, rather than directly assist them. They did this by exploiting various "loop-holes" in the [Heaven System]. And after they actually left Heaven and joined the Khaos Brigade, they became Fallen Angels.

So it's likely that It's very difficult, if not impossible, to write a Pure Angel as an antagonist. If the Pure Angel directly acts as the villain, he'll be recognized as an "enemy of Heaven", and they'll automatically become Fallen Angels.