You can see both mechanisms of survival in the human being: the instinct of survival of the individual (egoism) and the instinct of survival of the species (altruism). They both are present in every human being, and they are "transfered" from one human being to another in a "contagious" manner. When somebody is under the control of selfishness, he is virtually a hero for himself, and a killer for everybody else. There you have Cameron, a non-violent Buddhist man whose life you get to find caught under selfishness, with no fear to be killed by a gun, maybe even wishing to be shot as a sign of compassion for himself, who's willing to stop his suffering after other people "transfered" so many negative emotions to him that now that instinct is taking his life under control. And you can find that apparently hateful policeman, John Ryan, who abused a woman in the first minutes of the movie, saving later her life after a car accident, in this case, under the control of altruism, and acting as a hero, not for him, but for that woman. It's the perpetual fight of Good and Evil, depicted in a precise and understandable way. The policeman's father has a painful disease, and he transfers all his anger to his son day after day, who feels each day a little more helpless about a former compassionate person he truly loves. When we get to know that fact, we suddenly see him under a completely different point of view. The movie seems to tell us, "you see? it's just a matter of knowledge", and we even understand his madness: as spectators, we don't catch his anger any more. Fear, anger, desire, attachment. You can see all negative emotions which, according to Buddhism, cause suffering to humans, which in turn makes our ego grow and veils our minds. Other movies, like Star Wars, show that "force", and also how easy it is to fall into "the dark side", but no movie showed before, without metaphors, and with so much detail, how human nature works.
Suddenly, in the middle of madness, a combination of coincidences makes a miracle happen: a gun doesn't kill a little girl. That's a drop of Good which turns upside down a whole ocean of Evil, and shows that altruism, illusion, positive emotions... Good, in a nutshell, is our way to go, and also the strongest tool for survival provided by nature.
And this latter "miracle", and other scenes in the movie, like the one where Jean Cabot is angry and doesn't know why, and one minute later falls down the stairway, make me see this movie even in a deeper way. The Celestine Prophecy comes to my mind, with not just instincts involved, but the universal energy (something like "qi" from Eastern philosophies) flowing among humans in the form of positive and negative energy, and a superior ego controlling each of our lives in every detail as a master puppeteer. Within this framework, that miracle wasn't just a coincidence: it was one part of a kaleidoscopic reality where universal energy forces coincidences to happen. In other words, the little girl, with her innocence and true love, made that possible through a change in what we know as reality. This concept is very close to Buddhist karma.
Finally, this movie can also be interpreted from a scientific point of view in the form of the theory postulated by Richard Dawkins in his book "The Selfish Gene".
In any case, it serves as a hard criticism of the actual selfishness present in the American capitalist society, "preached" by Ayn Rand in her bestseller books, which have got to be almost as popular as the Bible, a line of thought that is spreading from the USA together with psychoanalysis to other countries like France and Argentina as a cancer in society. There are older movies which cover this issue in a metaphorical or allegorical way. One of my favorite ones is Invasion of the Body Snatchers (1956), remade in 1978.
Crash is a great movie, worthy of the Oscars it got. Watch out, there are two movies with this name in 2004.