If life isn't worth creating, why is it worth preserving? Why not take this to the logical conclusion of voluntary total extinction? Furthermore, how could one even discuss good or bad without a frame of reference?
Benetar argues death is often worse than life. It is non-existence that is better than both.
Non existence comes after death. Death can be painless. How, then, is death worse than non-existence?
These are terms used in a philosophical argument, so you have to be precise about definitions. In this case, non-existence is not possible once life begins. Non-existence is the alternative to creating new life. So , as far as we are concerned, in this context, non-existence is not considered to be what occurs after death.
Death is not just a simple transition from life to non existence. Death has a whole bag of unpleasantness associated with it. Fear, angst, pain (in theory it can be painless, in practice it's not that easy) and perhaps most importantly sadness and a feeling of loss among the people you leave behind.