Why does Ruby fire her Berriger (thereby shooting King in the neck) at the end?
At that point, both King and Elmore have tried to kill each other, but are unable to go through with it. Elmore shoots a bullet into the ground, at which Ruby cries "Elmore!" and rushes out. And then she shoots.
Apparently, King's death is a complete accident. But why did Ruby shoot in the first place?
"Y'know, I think Bertolt Brecht was rolling in his grave."
-Nellie McKay on the 2006 Broadway production of The Threepenny Opera, in which she played Polly Peachum
I saw the Signature production a couple of years ago. If I remember correctly, she rushes out because she thinks that Elmore has shot King. She intends to shoot Elmore but as a result of her rushing out and shooting so quickly and without looking, she kills King.