An animal killed by a predator or that died naturally still had blood in its tissues. This made it ceremonially unclean and a possible source of ceremonial uncleanness (Leviticus 22:8). Israelites must throw such an animal to the dogs or give or sell it to a non-Israelite (Exodus 22:31; Deuteronomy 14:21).
If an Israelite ate an animal with undrained blood, he or she became ceremonially unclean. Then they had to wash in water. The uncleanness was not a sin, but ignoring it led to punishment.