It's open to a significant verse from Psalms 22: "Deliver my soul from the sword; my darling from the power of the dog." Inferring The Power of the Dog's title meaning, it's implied that Phil is the dog and that Rose, or even Peter himself, is the darling being delivered by the poisoning plot.
Referring to the idea that he is projected as a Christ-like figure, it could be concluded that Peter is the “darling,” as Christ was the darling of God and that by The Power of the Dog's ending, Peter is free from Phil's power. However, Christ was freed by sacrifice, not by murder.