You know, this has no root in the historical, but about 5-6 years ago Charles Vess (an artist) did a series of comics call "Book of Ballads and Sagas." In it, it was William who had been bewitched, and the only was to save him and ensure he wasn't damned forever was for Barbry to deny him her love - even tho she really loved him. Once she had saved him, she herself died, from a broken heart and the emotional strain. There's nothing in the song to suggest it, but I like this version best. If you sing it "Out of ONE grave grew a red red rose, and out of the OTHER a briar" it leaves it open as to who was the false love and who the true.