Here is a layman’s attempt at understanding
Thomas Nagel’s argument that Death is a bad thing.
Nagel argues that death is not bad in itself, but since death deprives you of life, it means that it is indirectly bad. Even if a life is full of misery? Yes, because even misery is an experience. Death is devoid of experience, so even a life full of misery is preferable to death.
Some philosophers in the past have argued that the fear of death is irrational since there won’t be any post-death experiences. You cannot be afraid of something that doesn’t exist. As Epicurus put it: 'Where death is, I am not; where I am, death is not.'
Another argument put forth against the fear of death is that it doesn’t matter whether you die young or old, since you will be dead forever either way.
A third point is that your nonexistence after death is just a mirror image of your nonexistence before your birth. Why should you fear one more than the other?
Unfortunately, Thomas Nagel criticizes all three arguments against the fear of death, in his essay ‘Death’. ‘Suppose’, Nagel says, ‘an intelligent person has a brain injury that reduces him to the mental condition of a contented baby. Certainly, this would be a grave misfortune for the person. Then is not the same true for death, where the loss is still more severe?’
The second argument, that dying young is not worse than living long, is just as poor. Being alive and having experiences is what is good about life. If a person dies prematurely, she is deprived of those experiences. Being dead is neither good nor bad since it is devoid of experiences. Therefore, what is evil about death is not the state of being dead, but the loss of life. More of life is better than less. Bach had more of it than Schubert because he lived longer.