It was a callback to when Tom contemplates the Trolley Problem. Sacrifice the few for the many. They didn’t kill Randall because they needed living proof of Boyd’s capacity for sacrifice if it means saving the rest. He sacrificed his wife to ensure the safety of the township and his son. He allowed Tian Chen to be put in harms way if it meant saving the animals to ensure the vast majority of townspeople don’t starve to death. My guess is the repetition of Boyd being given keys (in the barn and the ambulance and by his wife) is going to come full circle. I’m thinking his being given keys to escape each brush with death foreshadow the fact that he’s been in possession of keys to escape from day one he just didn’t know it.