This right here is what we call an oxymoron. Putting side by side two opposite in meaning words, yet they complement themselves. It usually has a very special way of describing a character or thing. You would be making a huge mistake giving these words a literal meaning.
The sympathetic Devil is in no way a thing because we know the Devil is far from being sympathetic, in fact if it seems sympathetic then you should beware because there is a trap somewhere in between.
It can also describe a person who is very cruel but at a point decides to do someone a favour against all odds, this will be a good way to describe such a scenario.
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