We must die in order to be remade - to be resurrected.
This is salvation.
Damnation is the other choice: to reject salvation.
We must be resurrected to enter that state called Heaven*; because for Heaven to be 'heavenly' - all within it must have-been remade, wholly-good - without disposition to evil.
Or, to enter Heaven, we must make a permanent commitment to repent and repudiate all sin - all evil, all that opposes divine harmony - and the way that this permanent commitment is made is by resurrection -- in which all that is Good (God-harmonious) in us is retained; while all that is not is left-behind and discarded.
Such a permanent commitment is made from love; and therefore must freely be chosen - cannot be compelled; and can only be made by those capable of love who choose to make love their eternal foundational principle.
(Only thus can Heaven be a place that is wholly Good (a place without any evil-motivation) and also wholly-free - inhabited by beings with divine powers of creation; who will always and spontaneously use their godly-powers harmoniously with God and other Heavenly beings.)
The need for death is true for men and women - and for all other beings.
Which is why this incarnated mortal life on earth is dominated by entropy: because every-"thing" must die, if there is to be a possibility of resurrection.
In other words: If every being on earth is to have a chance of attaining and choosing Heaven - they all must die, sooner or later.
Pre-mortal spirit life, although wholly-good, is very imperfect - especially in terms of freedom, of agency.
The harmony and goodness of pre-mortal life is dependent on the passivity and obedience of spirits; because pre-mortal beings are not innately wholly good (in the way that God is wholly-good, or resurrected beings are wholly good).
Because pre-mortal spirits are not-wholly-good, the harmony of goodness in pre-mortal life is attained top-down, by the direction and control of God.
In effect, so far as pre-mortal spirit life goes; God is the wholly-good parents of a mixed bunch of children - some mostly-good, some mostly-evil - none wholly good. It is only by the obedience of these children that goodness prevails.
But pre-mortal spirits mature, they grow-up, they change... Sooner or later, they get to a point where they must throw-off the passive goodness of obedience to parental authority and control.
Then these pre-mortal spirits have a choice...
Either the spirits can incarnate on earth - some time afterwards to die, and make the choice of resurrection, or not.
(This is God's plan - because God wishes to make and inhabit Heaven with resurrected (incarnated everlasting) beings, who have have by their choice of resurrection made an eternal commitment to live in harmony with divine creation.)
Or else those pre-mortal spirits who do not want to die and be resurrected; or who simply do not want to die - and those who reject the divine plan for an harmonious Heaven of free-beings who have chosen to be remade without evil...
These spirits can escape their previous state of obedience to divine goodness, and enter the sphere of earth while still spirits - and therefore immortal.
These are the demons.
(This explains why demons are spirits - not embodied; and why they are immortal. They are spirits because they have refused temporary mortal incarnation, and are immortal because they are spirits, and unaffected by 'entropy'.)
If the pre-mortal spirits choose mortal life on earth, they must choose temporary incarnation and death (as necessary pre-requisites for resurrection); but if they reject this package, then such spirits have rejected even the possibility of Heaven.
This is why all demons are evil, and why demons are worse than incarnated Men - because demons, by their rejection of mortal incarnation, have all chosen to reject the possibility of that death which makes salvation possible.
(If demons changed their minds and repented, and wished to prepare for the choice of Heaven; they would need first to incarnate and die. Whether this this is possible or ever actually happens I do not know.)
So - demons have rejected the passive ('secondhand') good of pre-mortal spirit life under the parental influence of God; and they have also rejected that mortal life on earth which is a necessary stage to prepare for remaking-by-resurrection, and Heaven.
The ruling principle of creation (that which makes creation cohere) is love; and love is the reason why the dead choose resurrection and heaven.
The essential reason why demons have rejected mortal incarnation, and why some dead mortal Men reject the offer of resurrection into Heaven and instead choose damnation; is that demons and the damned are either incapable of love, or have rejected love as the basis of life and chosen... something else.
Differently phrased: Demons are those never-incarnated beings who have rejected living under the domination of entropy (i.e. mortal earthly life); and by doing so rejected God's plan of salvation.
The damned are those who chose to live and die as mortal incarnate Men on earth, and have become dis-carnated beings (i.e. souls severed from their bodies); and who die and then reject resurrection and Heaven; instead choosing some other fate.
*Note: Why our final God-destined state-of-being is embodied, incarnate - rather than spirits - is a topic I have considered elsewhere.