No reason it should be a problem.
Conceptually, this could be a tiefling who has been run out of town so often that she finds greater peace and quiet among the wilderness than among the people.
It could also be very reminiscent of a classic "made a pact with the devil" kind of witch -- she lives in the forest and dances naked with demons in the pale moonlight, in exchange for secrets of the wilderness.
I'm also reminded of Pan, a god of nature turned to a god of evil. Dark woods with dark lusts, plants feeding off the blood of sacrificial victims, a sickle brought against civilization.
Perhaps this is atonement for the ancient pact with Asmodeus, trying to restore the balance of nature and put tieflings back into the natural order from whence they came, purging the evil with natural spirits. Druids are shape-shifters, after all, perhaps a tiefling can feel freer, less stained, when they assume the form of a pure white dove or a majestic eagle.