Werewolves, which Remus is, become bloodthirsty and merciless wolves on full moons. Their aggression does not apply to animals (or animagi in the form of animals, which are Peter, James and Sirius, his friends), but it does apply to people. Severus is the enemy of these four, and at the same time also the person to whom Sirius offered to come to the shrieking shack as a Prank, where Lupin waits out the full moon. And if nothing is done, he will be devoured now. James will try to save him