Moral evil is only possible for a being that need not think about all it knows. Requiring a diversity of concepts in order to know suffices to make some item of knowledge known but not thought about. But any finite intelligence – whether angels or human beings – requires a diversity of concepts in order to know.
The blessed cease to know by a multitude of concepts so far as God himself becomes the concept by which they know, even if they still traverse this concept by some sort of discourse and therefore multitude. The eschatological conquest of moral evil requires that all evil be somehow subordinated to this sort of vision.