HE SAW TOO MUCH AND OR
REFUSED TO COME.
I actually rewound the scene to doublecheck what happened.
They BOTH shot him. Hayes got the final killing shot as they were struggling on the ground.
They already suggested that it may have been an accident. If it wasn't an accident, then it was that the boy came back and saw more than he was supposed to and they couldn't leave any witnesses.
I could buy that he saw too much since he was older and probably understood what was going on. I have no idea how that wouldn't affect the girl though, but I guess we don't really know if it did or didn't.