In keeping with wolfish tradition, it's an agonizing process, which may account for the creature's ill temper upon becoming fully lupine: He doesn't kill to feed any need besides bloodlust, and his eviscerations are graphic, grotesque and largely unnecessary although when Mr. Del Toro is seen chowing down on some villager's liver, it may be intended as an homage to his somnambulistic co-star Mr. Hopkins (hold the fava beans).
WSJ: Film Reviews: 'Wolfman,' 'Valentine's Day,' 'Ajami' and 'Red Riding Trilogy'