The phrase "for in His image did God make him" comes to explain why God treats animals' blood so differently from human blood. Animals may be killed to serve the needs of human beings, but human beings may not be killed to serve the purpose of animals or of other human beings, due to the unique spiritual quality man possesses, the "divine image" within him.
We discussed a verse in Parashat Noach (9:5) which appears to indicate that God holds animals accountable and punishes them for killing human beings. We encountered a number of different views among the commentators as to whether the doctrine of reward and punishment applies to animals. The Ramban, as we saw, seems to deny this concept, but does acknowledge the possibility that a special "gezeirat ha-Melekh" – divine decree – requires that animals be killed for killing human beings. Although animals are not subject to the doctrine of reward and punishment, since, after all, they do not possess the knowledge and understanding to choose between right and wrong, a special rule ordained by God dictates that an animal must die if he kills a human being.
Although the Ramban does not provide or allude to any reason for this "gezeirat ha-Melekh," we might venture to guess that it stems from the very next verse: "Whoever sheds the blood of man, by man shall his blood be shed; for in His image did God make him." The Seforno comments that this verse comes to explain the distinction drawn in the previous verses between human beings and the animal world. Recall that this section began with God's granting of permission to Noach and his descendants to partake of animal meat. From there God proceeded to clarify that although man may kill animals for food, they may not kill other human beings, and, according to some commentaries, even animals are forbidden from killing humans. The phrase "for in His image did God make him" comes to explain why God treats animals' blood so differently from human blood. Animals may be killed to serve the needs of human beings, but human beings may not be killed to serve the purpose of animals or of other human beings, due to the unique spiritual quality man possesses, the "divine image" within him. Whereas animals are purely physical creatures, the human being is endowed with a Godlike dimension which sets him apart from the rest of God's creations.
This might explain why, according to the Ramban, a special "gezeirat ha-Melekh" was issued sentencing to death any animal that kills a human being – to highlight the basic difference between the blood of humans and that of animals. As the Ramban notes, animals cannot discern between right and wrong and are thus not subject to punishment. But a killer animal is killed not as a punishment, but to underscore the fundamental difference between man and beast. When an animal kills a person, the impression is given that just as men kill animals and animals kill other animals, so do animals kill human beings, as part of the broader struggle of all living creatures for survival. God therefore proclaims that He will exact retribution from animals that kill people, for the human being stands separate and apart from the rest of creation. Even if animals do not and cannot "deserve" punishment, God decrees death upon an animal that kills a person so as to emphasize the significance and sanctity of the human soul and its distinction from all other creatures on earth.