The allegory of waxing guilt
This story is not about a bully at the beginning, but somewhere along the way, the book becomes the story of a bully doing the wrong thing. At first, Crash is not evil. In fact, it is easy to understand and sympathize with him. The "good" guy seems to be this Penn kid who is only ever perfect and always has himself together. Then again, Crash's family life is not nearly as supportive as Penn's, so Crash is jealous. It is because of his frustration and jealousy that he takes the path that makes him—for a time—into a villain.
The archetypal victim
If Crash becomes the archetypal villain (a hurt person hurting people), then Penn is the archetypal victim. As much as Crash wants to point the finger at Penn, like Penn is harming him somehow, the truth is consistently that Penn is a good person and Crash is punishing him for his privilege, even when Penn is using that privilege in only the best ways. Penn is so archetypally good that when Crash finally works through his own personal beef, Penn willingly becomes his friend and has already forgive him.
The evil foil
In some ways, Penn and Crash are absolute foils, but that is not the only foil for Crash's character. In fact, this Mike Deluca character serves as a nice foil for Crash's character. Remember a foil is someone basically similar in most ways, but obviously different in a few specific and meaningful ways, so Crash's other foil is Mike Deluca, because Crash is a basically good person who does some bad sometimes, and Mike is similar to him in every other way, but he is basically an aggressive and punishing person who does some good sometimes. Crash's character is exposed by that friendship.
Patricide as a symbol of guilt
In symbolism that is so painfully obvious so as to be contrived, we see Crash's willingness to be violent and harmful in its most essential, archetypal format. Crash literally tackles his own grandfather in a game of family football. The faux pas reveals about his character that his anger is essentially misanthropy, which is a very dangerous path! Because he has hatred for some people, he is essentially defined by hatred, and not even his closest loved ones are safe from the person that makes him into. Also, old men are often symbols for God, which means that he is angry about his fate. This is well-known symbolism in literature.
The union of opposites
When Penn and Crash become friends, their union of opposites is more than just them becoming friends. Crash has learned the missing aspects of his character that make him more like Penn, and Penn has also bended himself toward the masculinity of competition and fraternity. He warms up really quickly to women, but now, Penn is able to reach across the gap and have respect for Crash's competitive and aggressive ways. Crash lets him win, an excellent use of symbolism, showing the final reversal of their dynamic.
