Lyric discussion by Busker666 

Cover art for Matty Groves lyrics by Fairport Convention

It's interesting that technically there was only one murder in the song. Lord Darnell's killing of Matty would have constituted a legal duel, generally frowned upon by the authorities, but not illegal.

It's likely that neither constitute statutory murder. Matty was a peasant, and killing him under any circumstances would not likely have brought a charge to a nobleman. Both killings in the specific circumstances were probably justified in law, since discovering a man with your wife if you were noble in many places warranted the murder of both the wife and the lover. Certainly, in Ancient Greece for instance, the murder of the wife was encouraged, and the murder of the lover forgiven if the man of the house chose to do it.