Darren is a high school student with a best friend, Steve, who gets him in trouble, and who experiences existential angst in a typical suburban family that repeats the mantra of boredom to him, “a productive life. College, job family, college, job family.” His dread is compounded by the belief that “we’re all gonna end up in the same place (dead) whether we like it or not.” When he visits a circus freak show in his town, he discovers it is led by a “good” vampire who doesn’t want to kill people, just suck their blood a little. Turns out that a band of “bad” vampires who still kill people are trying to start a war with these “good” vampires in order to get some real blood spilt. Darren becomes a half vampire in order to save his best friend from dying, but unfortunately that friend gets caught up with the bad vampires. So we now have best friends on opposite sides of this building war.
But the real theme behind this story is that of determinism and free will, namely, that biology is not destiny. We create ourselves through our choices: Existentialism. Early on, we hear Steve talk about his “bad blood,” which is confirmed by the good vampire later. And as he says that, we are taken on a cinematic journey into his very blood cells to see that there are evil “elements” in his DNA. We hear the phrases, “Destiny, we all have a destiny,” “It’s all been written down in a book.” After Steve goes “bad vampire” he tells, Darren, “Too late, I have my destiny, you have yours.”
However, the tension of free will is woven in throughout as we hear the lead vampire, John C. Reilly, state, “I may write the script, but the characters seem to do what they want.” And Darren is told, “You seem to have a lot of free will” because he is able to avoid drinking blood even though it will ultimately kill him. Finally, when Darren struggles with whether or not he will become bad because he is a vampire, his monkey girl girlfriend tells him the theme tagline: “Being human is not about what you are, it’s about who you are.” Darren concludes, “Just cause I’m a vampire, doesn’t mean I’m blood thirsty.” Thus the film argues against Original Sin in that it concludes we can have control over our natures to do good or evil and there is no inherent evil to our nature, merely natures that can be used for good or evil (thus, the good and evil vampires, a similar concept to the Twilight series).
Another way of looking at it: “What you are” is an argument for identity being rooted in an essence or substance (biology), but “Who you are” is identity being rooted in personhood or free will.
There is some comfort here for those of us who consider ourselves freaks in society and don’t fit in as we are shown that everyone has their place and special purpose to perform, though of course, we must choose to do so.