JRR Tolkien quoting - with agreement - Simone de Beauvoir:
[De Beauvoir]: "There is no such thing as a natural death: nothing that happens to a man is ever natural, since his presence calls the whole world into question. All men must die: but for every man his death is an accident and, even if he knows it and consents to it, an unjustifiable violation."
[JRR Tolkien]: Well, you may agree with the words or not, but those are the key spring of The Lord of the Rings.
On the one hand, death is universal and thus, apparently, natural; on the other hand, death is also experienced as profoundly un-natural, accidental, a violation.
Traditional Christian theology has attempted to deal with this using the concept of Original Sin; but I find this unsatisfactory - both for having been (I would have thought obviously) inserted post-Jesus and therefore not genuinely Christian; and also because Original Sin theory fails to do what it claims, which is to explain the prevalence of evil among Men without implicating God.
Instead; my understanding of death is that it is experienced as both natural and unnatural because of our situation in mortal incarnation - which I regard as (for Christians) situated between a potentially deathless pre-mortal spirit existence; and the post-mortal incarnated state of resurrection.
This mortal life of ours is temporary, a phase not an end-state - but our basic expectations deriving from pre-mortal spirit life are that we 'ought to be' eternal and deathless. Furthermore, since the life and work of Jesus Christ, Christians have hope of an eternal and deathless state to follow this mortal life.
Yet, the actuality of this mortal life and its inevitable termination - is unnatural. It is also not under our control; since other factors (primarily God, but also Men and other Beings and happening) influence how and when we die.
We cannot, therefore, take death for granted. Death comes, and will be a time of transformation. It is a severing of soul from body, as the body dies - and (because we are incarnate Beings) the body's death changes us, removing part of our-selves - and what remains after death is naturally-speaking incomplete.
In other words; the spirit after death - which has been variously conceptualized throughout history - is significantly like a different person, and (it has been believed) is often severely diminished in its coherence, identity, agency etc.
I think we sense exactly this (mostly implicitly), when we consider death. It probably lies behind the yearnings for 'peace' after mortal-death, which are so often the wish of non-Christians (including many self-identified Christians who do not want the resurrection that Christ actually offered).
...Anyone who felt sure that death will certainly annihilation would not be concerned to ask for peace; anyone who was confident that death was naturally a peaceful state would not feel compelled to pray for peace.
There is a fear (clearly expressed by Hamlet in his famous "To be, or not to be" soliloquy) that after our death we will experience a state of inescapable nightmare; and that this may be the 'default' condition - unless... something else happens, or we take some particular actions or make choices.
Thus, from our perspective here-and-now in mortal life; death is indeed the threat of a violation that seems unjustifiable; unless made-sense-of by resurrection - or some other desired outcome. And that death seems to be non-optional makes matters worse.
Original sin was and is an attempt to make sense of this, but since it does not work then we need something else - and the explanation ought to be a clear and graspable kind of truth (as indeed it surely would be, given the nature of our God).