In the tragedy "Faust" by German poet Goethe we hear Mephisto "The Devil" say the words: "I am part of that power which eternally wills evil and eternally works good.� What do you think is the meaning behind that?
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Never read it, but I'd say that, on an ultimate, metaphysical level, everything always works out for the good. What we call evil is only a human, subjective judgement. The negative aspect of an act only exists on the surface; there are two sides to the coin, and the side that works good is subtle and isn't easily visible to us (if it were, it wouldn't be one of those things we call "evil.")
The devil does things that would normally be classified as evil and atrocious, but he knows somehow that what he's doing ultimately serves us all. Maybe he even consciously works toward this, even though he does this by unpopular means. He does the things nobody else is willing to. Or maybe he just has extreme personality traits, maybe hatred or anger issues or maybe something else, and he soothes himself by knowing that everything is ultimately productive and good in God's eyes.
In one of the books in Neale Donald Walch's Conversations with God series, God says that if it weren't for evil, we couldn't know of ourselves as good.
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By "or maybe something else", I mean maybe he has some pathological drive that causes him to want to do damaging things on a continuing basis, and either it's so strong that he's unable to control it, or he figures that it's best for him to just "do him" than to die slowly by suppressing himself, even if he recognizes his own pathology.