Clarity will be served if we distinguish the specifically Epicurean reason for thinking death not an evil from another reason which is actually anti-Epicurean. I'll start with the second reason.
A. Death is not an evil because it removes us from a condition which on balance is not good, a condition which on balance is worse than nonexistence. This is the wisdom of Silenus, reported by Sophocles (Oedipus at Colonus, ll. 1244 ff.) and quoted by Nietzsche in The Birth ofTragedy, section 3:
There is an ancient story that King Midas hunted in the forest a long time for the wise Silenus, the companion of Dionysus, without capturing him. When Silenus at last fell into his hands, the king asked what was the best and most desirable of all things for man. Fixed and immovable, the demigod said not a word, till at last, urged by the king, he gave a shrill laugh and broke out into these words: "O wretched ephemeral race, children of chance and misery, why do you compel me to tell you what it would be most expedient for you not to hear? What is best of all is utterly beyond your reach: not to be born, not to be, to be nothing. But the second best for you is -- to die soon."
B. Death is not an evil for the one who dies because when death is, one is not, and when one is, death is not. My being dead is not an evil state of affairs because there is no such state of affairs (STOA) as my being dead. Since there is no such STOA, there is no bearer of the property of being evil. If this property has a bearer it cannot be an individual or a property but must be a STOA.
And so the Epicurean line is consistent with life affirmation. The Epicurean is not saying that being dead is good and being alive evil; he is saying that being dead is not evil because axiologically neutral. The Epicurean is therefore also committed to saying that being dead is not a good.
The first reason is axiological, the second ontological. The Silenian pessimist renders a negative value verdict on life as a whole: it's no good, better never to have been born, with second best being to die young. By contrast, the Epicurean's point is that the ontology of the situation makes it impossible for death to be an evil for the one who has died.
This reinforces my earlier conclusion that there is nothing nihilistic about the Epicurean position.
From Some of the Dharma, Viking 1997, p. 175, emphasis added:
No hangup on nature is going to solve anything -- nature is bestial -- desire for Eternal Life of the individual is bestial, is the final creature-longing -- I say, Let us cease bestiality & go into the bright room of the mind realizing emptiness, and sit with the truth. And let no man be guilty, after this, Dec. 9 1954, of causing birth. -- Let there be an end to birth, an end to life, and therefore an end to death. Let there be no more fairy tales and ghost stories around and about this. I don't advocate that everybody die, I only say everybody finish your lives in purity and solitude and gentleness and realization of the truth and be not the cause of any further birth and turning of the black wheel of death. Let then the animals take the hint, and then the insects, and all sentient beings in all one hundred directions of the One Hundred Thousand Chilicosms of Universes. Period.
Nature is the cause of all our suffering; joy is the reverse side of suffering. Instead of seducing women, control yourself and treat them like sisters; instead of seducing men, control yourself and treat them like brothers. For life is pitiful.