I go back and forth on this question. I should be ashamed of myself. Forty years a philosopher and no fixed view on such a fundamental question? What am I (not) being paid to do? To gain some clarity, I will sketch some possible views. I will also sketch the view to which I incline (despite my vacillation).
But first I define 'mortalist.' A mortalist is someone who holds that we human beings are mortal, i.e., subject to the natural necessity of dying, both in body and in mind. Accordingly, all human beings will eventually die, and when they do they will utterly cease to exist as individuals, even if they persist for a while after death as corpses or as smoke and ashes. (By the way, I consider transhumanist dreams of immortality here below to be the worst sort of self-deluding, ultra-hubristic sci-fi nonsense. Pox and anathema be upon this house of cards.) For the mortalist, then, as I define the term, there is no natural immortality, as in Platonism, nor any supernatural immortality via divine agency as in Christianity.
A. Views According to which Death is not an Evil
1. The first view, that of the pessimistic mortalist, we can label 'Silenian.' On this view, 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, if wisdom it is, of Silenus, reported by Sophocles (Oedipus at Colonus, ll. 1244 ff.) and quoted by Nietzsche in The Birth ofTragedy, section
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."
Better never to have been born, but here we are. So second best is to die as soon as possible. Death is not an evil, but a good, since it releases us from an evil condition, that of being alive.
2. The second view is that of Epicurus. On the Epicurean view, 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 for me (though it may be for others) because there is no such state of affairs (STOA) as my being dead. There is no such STOA because when I am dead there is no bearer of the property of being dead. And there being no such STOA entails that it it cannot be an evil STOA, or a good one for that matter.
I must point out that some find this reasoning sophistical. Well, if it is, is is not obviously sophistical. Some of the complexities of the reasoning are explored in a number of posts collected in the Death and Immortality and Epicureanism categories. I can't go into this now since this post is mainly just taxonomic.
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. It is not evil because it is axiologically neutral. The Epicurean is therefore also committed to saying that being dead is not a good.
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.
3. Platonism. For the Silenian, death is not evil because it releases one from life, which is evil. For the Epicurean death is not evil because the decedent is nonexistent, hence removed from all goods and evils. One cannot experience loss, or suffer in any way, if one does not exist. On the Platonic view death is also not an evil but for a different reason: death is release of the naturally immortal soul (the person in his essence) from embodiment. From a sub-standard 'cave-like' existence, the soul is freed to enjoy a true existence. On Platonism, the true self continues to exist post mortem in better conditions.
4. Illusionism. Whether or not actually held by anyone, there is the possible view according to which dying and being dead are illusions. If so, then how can they be evil? The enlightened sage sees through the veil of maya and recognizes his true identity as the deathless Atman (=Brahman). We don't exist as separate individuals and we don't die as separate individuals. I am the eternal Atman, and as such deathless. Moksha, enlightenment, liberation, is to realize my identity with the eternal Atman thereby seeing through the illusion of separateness. For some puzzles relating to moksha, see here.
5. The view to which I incline. Although the process of dying for most of us won't be easy, physically or mentally, the evil of dying is outweighed by the good of being dead, the good of being released from a predicament which is plainly unsatisfactory, whether or not we survive our bodily deaths as individuals. One aspect of the unsatisfactoriness of our present predicament -- and it is indeed a predicament -- is our deep ignorance, an ignorance that in some takes the form of delusion. (We are de-luded, played for fools, by a world which obtrudes itself upon us as the ne plus ultra of reality when calm reflection shows that it can be no such thing.)
If you deny that this life is plainly unsatisfactory, and can in the end offer us nothing that truly satisfies, then you live on a different planet and I can't help you except to refer you to Buddha, and the Preacher of Ecclesiastes, and Plato, and Augustine, and Thomas a Kempis, and Schopenhauer, and a thousand other philosophers and sages East and West.
Mine is not the position of the pessimistic mortalist, the Silenian, because I am neither an out-and-out pessimist nor a mortalist. Life is not thoroughly bad, but a mixture of good and bad, a chiaroscuro of axiological light and shade if you will. It's not all night and fog; there is daybreak and sunshine and thus intimations of Elsewhere. And if this life is a vale of soul-making, as I am inclined to think, then it is instrumentally good.
Mine is not the Epicurean position because I am not a mortalist.
Mine is not the Platonic position because I do not dogmatically affirm the immortality of the soul. (By 'Platonic' I do not mean the actual views of Plato, whatever they were, but something much broader and caricature-like.) I maintain merely that belief in it is rationally acceptable. The rationality of the belief supports the hope that we may come to learn in death what we cannot learn in life. On this view death is not an evil but an adventure into Shakespeare's "undiscovered country from whose bourn no traveller returns." (Hamlet's soliloquy.) Death is an adventure, and one to be embraced and prepared for, given that one has perceived that this world has nothing much to offer us.
The poet and drunkard Dylan Thomas had it exactly wrong when he advised not going gently into that good night but raging, raging against the dying of the light. I liked his famous lines (which I did not just now quote but paraphrase) when I was an adolescent, but I have put aside childish things.
Peter Lupu once asked me why, if I believe that being dead is good insofar as it is a release from this unsatisfactory predicament, I take such good care of myself. My answer follows from what I have said. This vale of tears is also a vale of soul-making. So I need to 'do my time.' (Here, in nuce, is an argument against suicide.) I need more time here below to earn merit and make up for earlier transgressions. I need more time to complete my philosophical projects and prepare for death. No reasonable person embarks upon a long journey to a foreign land, there to take up permanent residence, without adequate preparations. How foolish, then, not to prepare for the journey to Shakespeare's "undisovered country"? You say there is no such "undiscovered country"? Well, then you need to inquire into the grounds of your belief. Or do you hold beliefs about matters of the utmost importance thoughtlessly?
B. Views According to Which Death is an Evil
6. Optimistic Mortalism. Death is an evil because life is unqualifiedly good and death deprives us of it. Does this need refutation?
7. Christian Mortalism. Death is an evil because we were intended to live in an embodied state forever in paradise with God. But now we are under sentence of death due to Adam's sin. Death was not intended by God but is a punishment for Adam's sin. Death, though an evil, is yet a portal to eternal life for those who accept Jesus as savior. So Chrisitan mortalism is not mortalism full-strength as I defined it at the outset, but a mitigated mortalism which pins its hopes on supernatural divine agency and the resurrection of the body.