The most obvious law of behavior that Polyphemus violates is hospitality. In Ancient Greece, and much of the rest of the ancient world, hospitality was much more important. Journeys took much longer, and there weren't nearly as many inns for travelers to stay in, so it was much more important for those with homes to take in travelers in exchange for some small gift or token of gratitude.
There are many examples of good hospitality throughout the Odyssey. Alcinoos and Arete, for instance, welcome Odysseus with celebratory feasting, thus proving themselves to be good hosts. Therefore, when Polyphemus abducts Odysseus and his men and begins to devour them, his violation of the rules of hospitality is much more obvious. After reading about much better examples of hospitality, Polyphemus' rude cannibalism becomes even more monstrous, and it is easy to side with Odysseus when he blinds the dastardly beast.