Well, the way I see it ...
The devil is an angel (spirit being). I believe that angels are created in God's image too, even more so than we are. They were created with self-awareness. Satan was the first of the angels to rebel against sub-ordination to his creator - he later lead other angels that way. I do not believe he is "evil" but the indirect source of "evil". This can only make sense if one believes in absolutes and the labels of "good" and "evil". Sin in Satan's sense was rebellion.
In the book of genesis you will read about the tree of knowledge of good and evil. I have come to the conclusion that this is a symbol for the instructions of Satan and his angels. Through these teachings man became self-aware and learned the capacity for judgement (the fruit).
Now man can judge for himself what is right and wrong.
An action such as murder is generally considered to be wrong, but some may be able to justify this action to themselves and so may be able to murder without consequence. This then leads the majority to judge that individual (or his actions) as evil. So evil is from the devil in the sense that he taught man to think for himself.
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