If my genes have inclined me to develop with a feeling that it's right to protect my family or wrong to kill someone, then how can I discount those feelings in order to become a nihilist without somehow 'ripping out' those genes and every effect they've had on me?
Or, to put it another away, knowing that my sex drive is only an adaptation produced by natural selection doesn't make me want to stop having sex, so why should knowing that my moral 'drive' is (at least partially) derived from the same source make me want to stop being moral?
Classic projection.
Pete apparently believes that he'd lose his moral sense and turn into an insensate psychopath or perhaps some kind of despairing nihilist if he weren't a Christian. Let's all hope for his sake and/or ours that he doesn't get a crisis of faith.