User blog comment:B214/Question and Answers 6/@comment-120.20.138.121-20170109132412/@comment-9595671-20170112030351

It's because they used that loop hole that they fell though, nothing says joining the Khaos Brigade in general will make them fall. I mean for one example, Teodoro was technically antagonistic for a while and hated Devils but he didn't fall.

So he's pretty much proof a pure Angel could indeed be a villain, they just can't betray the main teachings of God that Azazel and the first Fallen broke.