No, it's not murdering it, it's using it for something else Beethoven, Bartok, Brahms, Kodaly, Brahms, Schubert... all did it.
Probably the most sublime folk song turned into an orchestral piece is Butterworth's 'Banks of Green Willow' no longer folk in his orchestration, but stunningly beautiful.
The song is attributed to William Shield (1748-1829) and may not be a folk song anyway - it would depend on whether it passed into the oral tradition (the Roud index suggests that it hasn't).
'Old' or 'rural' doesn't necessarily mean 'folk', but then again, that's getting down in defining what you mean by the term - careful, you might upset the horses!!
Jim Carroll