Interesting comparisons can be made between versions of "The Suffolk Miracle" and versions of "Pretty Polly".
Demons and dead lovers became ordinary murderers on both sides of the Atlantic, in time, of course.

Changing times account for a lot of changes including the loss of Faeries, Dragons, Giants and Ogres. If a singer is not familiar with the belief system that surrounded the older ballad, they are easily dropped or replaced.

Their shadows linger in the old ballads, but they aren't obvious.
Cheers, Joy