After Nick came home, he was surprised to see someone, though the character in question remains offscreen.
Of course, there were several possibilities as to who the murderer could be, but the obvious suspect was clearly Beau, who was last seen grabbing his gun and storming out the door to find Nick after discovering he was back.
The shot happened in the very final moments of the episode.
Still bitter about Rooster’s apparent death at the hands of Nick, vengeance was on his mind.