The Null: It Only Gets Worse

If something like the Null as presented in Stephen King's Revival is real, no one who dies receives rest, reward, or pleasure of a moral or amoral kind; everyone is destined for misery after death.  The way the end of the novel describes the Lovecraftian hellscape, religious martyrs would be accelerating their entrance to a very different afterlife than they likely imagined or hoped for.  Soldiers, like the Spartans or Vikings, thirsting for the blood of others or eager to have their own lives ended in battle would be catapulting people, possibly including themselves, into something far worse than mere warfare.  People who commit suicide might be hoping for soul oblivion, and with it the eternal cessation of pain, only to find themselves forcibly summoned to the Null's slavery and torture, surrounded by the ant-like servants of Mother and her fellow eldritch abominations.

In the story, Mother exhibits telepathic control over many people exposed to a "secret electricity" that is underpinned by a greater cosmic energy present in the Null, driving them to suicide.  Suicide in other contexts can seem subjectively appealing to some people precisely because it ends suffering in this life, at the very least.  If there is an afterlife of any kind, there is consciousness after death, even if it is only reincarnation.  Consciousness, however, necessitates the possibility of suffering.  Without a mind that perceives, there would be nothing to have the capacity for pain, so whether a mind is integrated with a resurrected body (or a new body) or exists separated from a corporeal shell, the possibility of an afterlife by logical necessity entails the possibility of suffering.  Not all logically possible afterlives involve such agony, and certainly not the New Jerusalem of Revelation (the Biblical hell does not torment people endlessly).

The Null or any similar afterlife would still mean that no one who kills themself has really escaped torment.  They have only entered something presented as far worse than the most terrible things of earthly life.  Yes, the connections to Stephen King's multiverse in Revival, including the references to the locations of Jerusalem's Lot and Castle Rock and very probable ties to the Dark Tower of the literary ontology, mean the work is intertwined with stories like The Shining where there are other afterlives.  This means that not everyone goes to the Null upon death as the protagonist assumes (and he does merely assume it from a brief glimpse), the dimension is an illusion crafted by Mother to terrify characters Jamie and Charles (or one brought about by contact with the special electricity), or it is a temporary place where some or all people go to die again before potentially being sent to a better afterlife.  Jamie, who outlives Charles, still would not know what is the case until he dies.

Yes, it is also entirely possible that there really is an afterlife resembling what Jamie assumes the Null to be, unlike various ideas of a heaven or calm universalist transcendence.  Suicide might liberate people from suffering one way or another in real life, either by bringing about the end of the soul altogether or springing it into a blissful afterlife.  It also might not.  One could not know unless one was omniscient (or at least free of human epistemological limitations about the future and the afterlife) what, if anything, would await one's mind after the death of the body, whether nonexistence or a peaceful trek through matterless space or resurrection of the body where it is reunited with its soul or something more cruel than even the unbiblical misconceptions of eternal conscious torment in Yahweh's hell.  However dire and painful this life is, suicide cannot be proven to not lead to something far worse; either philosophical option is logically possible because neither contradicts self-necessary axioms, such as the inherent truth of one thing logically following or not following from another.

Pursuing suicide as a means of escaping this life's terrors is therefore not guaranteed to actually accomplish anything more than just that, which might lead to suffering of a much greater and longer kind.  In Revival, Jamie is told by Mother that the pain and subjugation will not lead to death or rest.  He has no access to the evidence seen in other novels by King that this afterlife is not universalist, that it is a total illusion, or that it is only a temporary afterlife, whichever combination of possibilities is applicable.  The novel ends with him awaiting his death.  "Come to me and live forever," Mother allegedly whispers to him.  As far as seems to be true to Jamie, suicide only figuratively throws him out of the frying pan and into the fire, where he will eternally reside.  It is not that the Null of all things is likely to exist despite the logical possibility.  It is that ending the pain of this life does not necessarily end all pain and we cannot have the opportunity to know until our own individual deaths.