Pamela R wrote:
"I do still find morbid or violent themes seem to be conspicuously prevalent, particularly familial murder (e.g., The Cruel Mother; Dowy Dens of Yarrow; The Cruel Brother; Lord Abore; Little Musgrave; Two Sisters; Willie Taylor; Young Edmund in the Lowlands; Lord Thomas and Fair Ellender)."

Just for interest, amongst the top 20 Child ballads (by number of variants collected) in Sharp's Appalachian MSS are:

Lord Thomas and Fair Ellender (1st)
House Carpenter (2)
Little Musgrave (6)
Two Sisters (10)
Lord Randal (12)
Young Hunting (13)
Two Brothers (14)
Cruel Mother (16)
Earl Brand (18)
Edward (19)
Sir Hugh (20)

Pretty Polly was the most popular of the non-Child ballads.

So I'm sure you're right that murder (especially familial) was a very popular theme in that place at that time.