Why not let the woman and kid go at the end?
I could understand if the husband was still alive and with them, not wanting to risk them leaving and coming back to kill you and take your food. But i mean Will is now dead and its just the wife and little boy, i highly doubt you'd have to worry about them coming back, i'm sure Kim would want to get as far away from them as possible and never come back.
I just didn't see the need in killing them.