I know, weird question, but there are a host of traditional, popular folk songs in which somebody gets killed. A few that come to mind:
Casey Jones
John Henry
My Darling Clementine
Tom Dooley…
How many more can you think of?
Matty Groves
Long Lankin
Sir Patrick Spens
Bonnie Susie Cleland
McPherson’s Farewell
I thought it was compulsory to have misery, woe, death and plenty of it!
Billy in “Stagger Lee”
Streets of Laredo
Barbara Allen.
Or Barbry Ellen if your’re a little bit country.
The Ballad of Jesse James.
The Ballad of Billy the Kid.
John Harty, the Murderin’ Man.
The Ballad of the Titanic.
Abdul the Bul-Bul Ameer.
The Pizen Sarpint.
Frog Went Courtin’. 
How far we going back?
If modern folk can count, I’ll vote for “Sam Stone”
Billy Jo(e?) McAllister.
I Shot The Sheriff. It sounds folk to me.
Lonesome Death Of Hattie Carrol - Bob Dylan
Would “The Wreck of the Edmund Fitzgerald” count as a folk song, or is that too recent?
There are two from The Doug Suite:
Doug at Home, the song about spontaneous combustion.
Doug at the Gates of Hell, the song right after the song about spontaneous combustion.
Does “Finnegans Wake” count, if he comes back to life at the end of the song? 
“John Barleycorn Must Die”
But it’s a good death 
Waltzing Matilda.
Wow. We need a definition of ‘folk song’ that includes singer-songwriters and reggae apparently.
How 'bout Woody Guthrie’s Tom Joad?
Cruel Sister
The Three Ravens
(site includes a translation into Dutch)
There is a big section on the Lomax compilation book (one of the best known Ethno-musicologists who chronicled all types of western folk music) that has the theme of murder ballads.
Captain Kidd (He gets hung for being a pirate) is probably my favourite one but there are many more.
If you go into the filk side of things you will find a lot of them have to deal with murder, oftentimes on a supernatural level.
Probably about 90% of Child’s ballads.
Lord Randall
The Golden Vanity
Barbara Allen
Anachie Gordon
Actually, sometimes you wonder how these people ever bred at all.
The Good Reuben James
The Long Black Rifle
Aura Lee
French Perfume
I know that just an ohnosecond after I post this I’ll think of a dozen more.