Having violent tendencies doesn't make something a folk song, and it doesn't stop it being a folk song.

That kind of argument is much the same as if someone were to say that a picture in front of them couldn't possibly be a photograph, because it showed something nasty happening, or even that it appeared the person taking the photo approved of the nasty stuff that was happening.

There's no use in trying to argue in a circle: "That song preaches violence. Folk songs don't preach violence. So that song can't possibly be a folk song."