score:4
The way you phrase the question seems to indicate that you think non-existence is nothing to be feared or is somehow not a punishment, but this is exactly what is to be feared. The vast majority of humans fear death, including those (eg atheists) who believe that death is equivalent to non-existence. When a criminal is handed the judgment of capital punishment, say by firing squad, the punishment is not the brief fraction of a second of pain from the bullet, but the ending of his life and deprival of any future joy. For Christians, our future joy is infinite, and is why the chief premise of the verse makes sense: do not fear the temporary death of the body, because we will be resurrected to be with God the Father; not temporarily alive but eternally. Rather, fear destruction, whereby you will be deprived of this; not temporarily dead, but eternally.
Edit: I should note that both this answer and the question are somewhat tangential to the point of the verse, which is simply that the 12 disciples should not fear men as they spread God's word, because they serve One far more powerful.