Death can harm you even though you never experience being dead
Nagel's essay on death confronts an old philosophical puzzle: if death means the total cessation of experience, how can it be bad for the person who dies, since there's no longer a subject present to suffer any harm? Ancient thinkers like Epicurus used exactly this reasoning to argue death should not be feared at all.
Nagel rejects this conclusion by defending what's become known as the deprivation account: death is bad not because of any unpleasant experience it contains, but because it takes away the good things — experiences, relationships, projects — the person would otherwise have had. He argues this kind of harm doesn't require a suffering subject any more than being deceived requires you to notice you've been deceived; both are bad in virtue of a comparison between what happened and what otherwise would have.
This matters because it reframes mortality's sting: the tragedy isn't a future state of being dead, but the abrupt foreclosure of a future that was, up until that point, genuinely open and full of possible goods.
Takeaway: what makes death bad isn't an experience you'll have, but the life you won't.