Sforno on Genesis 9:6/ספורנו על בראשית ט׳:ו׳

1באדם דמו ישפך, a reference to a human tribunal.

2כי בצלם אלוקים, this is the reason why G’d exacts retribution for killing human beings, whereas He does not exact retribution from man for killing animals, [even though killing, say for sport is prohibited. Ed.] The animals have not been created in the image of G’d, so that their death does not deprive the earth of a divine image. Only human beings are described in the Torah, on occasion, also as אלוהים, as for instance in Exodus 22,8. את האדם, from the moment G’d had said (Genesis 1,26) נעשה אדם, “let us make a human being,” He had endowed His creatures with an independent intelligence, able to defy Him either collectively or individually. It is this “divine attribute” of man that makes him sufficiently important for his Creator to demand an accounting from those who destroy that divine image by killing a human being.