We can understand Tetsuo’s transformation as one of circumstance.  But it is much more difficult to identify with Sin City’s cannibalistic, bible-reading Kevin—a boy that kills and eats women, leaving scraps for his pet wolf.  His resolute silence and cold impassive expression remove from him all that is human, leaving behind the pure embodiment of evil.  We cannot relate, but we can reluctantly consider the existence of such aberrations in our world.