The main idea of this film is that most times we consider someone a monster when we don't understand his or her journey. And it is revealed that the monsters of the film are men who seek power and money and are so greedy that they are willing to take lives in order that their own be extended. De la Guardia and his nephew, Angel hunt down Jesús, a man that has no desire for eternity but to merely live out his days in happiness with his family.
However, when the cronos pierces his skin and takes his blood, Jesús is beholden to the device and what it transforms him into. He is, by the end of the film, transformed externally into a marble-white skinned monster. This exterior though becomes the antithesis of who he is, a man of love and character. This shocking exterior is the manifestation of the internal state of the De la Guardia men who have lost their hearts and souls as they desire the things of the world while Jesús simply desires to live life until there is life no more.
Aurora becomes the character that breaks down the wall of this idea of being a monster as she welcomes her grandfather home after he has risen from the dead. del Toro is showing us that it is only when we see through the eyes of a child that we see the truth and are capable of loving one another. But, when we see people as a means to an end (De la Guardia seeing Jesús as a means to the cronos, and Angel seeing killing his uncle as a means to his fortune) we aren't living, but merely sucking all the air out of life from anyone that is close enough to be effected, or in these men's cases, used.
