Supposedly, say some, only the threat of torture without end could frighten us enough to do what is morally correct, as if justice, what people do or do not deserve based upon their beliefs, intentions, and deeds, could be a matter of whatever maximizes utilitarian intimidation. Setting aside all other logical and Biblical flaws with an afterlife of eternal torture being just, let us focus on this idea that anything short of dreading an eternity of suffering would never compel anyone to stop sinning. It is possible for people to believe or be motivated by something irrespective of its validity, so it would still not be absolutely necessary on a pragmatic level for all people to expect or fear burning for eternity (or any other torment) to desire to abstain from sin.
Few, if any, of these proponents would acknowledge the logical possibility of sheer sinlessness or that the Bible teaches this state is possible and that striving for it is obligatory (Deuteronomy 30:11-15, Matthew 5:48, and so on). They are wrong when it comes to this subject as well, but of more immediate relevance to the notion that eternal torture poses the only effective deterrence is the fact that many people who claim to believe an eternity of consciousness in punitive hellfire contrary to both logic and the Bible still sin. They are clearly not willing to stop sinning! Has asininely agreeing with infinite torture for finite sins ever compelled the evangelicals who erroneously think being without sin is impossible from giving up the most basic sins? Perhaps, but it is not the only possible means of deterrence, and not only is it unbiblical anyway, it is illogical.
Adding to their delusion, they think a host of nonsinful things are actually sinful, including neutral mental states like anger that might be entirely involuntary and thus could not be evil one way or another. They 1) believe there are far more sins than Biblical theology actually entails and 2) perhaps think some of these sins are inevitable. Ultimately, some foolishly think sin deserves eternal torture and that sin is all but unavoidable, and yet they continue to commit genuine Biblical sins like the conflating the sinful and nonsinful as aforementioned (Deuteronomy 12:32).
Since one must be irrational to believe either that it is possible for eternal torture to be justice (it is the greatest of disproportionalities, and contradicts the very idea that there is an objective line beyond which all treatment is unjust) or that the Bible teaches this fate awaits sinners (Matthew 10:28, James 1:15, etc.), it would not matter if an individual was subjectively fearful of endless pain to the point of not sinning. They would only experience subjective persuasion prompted by the threat of the ultimate injustice getting used as punishment for sins. And sin cannot be the just response to sin! Did the Bible teach the ultimate injustice is in actuality the punishment of God's righteous judgment, then, it would be in error on one of the most important philosophical issues after the inherent truth of logical axioms.
Certainly, nothing could be objectively worthier of dread than things within the hypothetical category of eternal torture, with some kinds of torture still being worse than others even if they all truly last forever. But the idea that only such terror is sufficient to deter sin is refuted by the subjectivity of experiencing fear (what inspires someone to act or not act sinfully could be vastly different for various people) and the fact that many who profess allegiance to the notion of eternal torture can and do still sin. It is also tied to the fatal flaw of utilitarianism: if something is evil, the ends cannot justify it as the means.
In addition to all of these things, this idea, heretical against both the necessary truths of rationalism and the plain doctrines of the Bible, trivializes the real pragmatic reasons to not sin on Christian philosophy. One should do as one should simply because it is morally right, but there are incentives. Is a blissful eternal existence in the very presence of God and Christ, with the total freedom to do as one wishes without sinning or pain (including that of boredom) and all that is entailed by this, too small a thing to inspire longing and commitment in moments of wavering? Though of course there are more terrible hypothetical fates than this, compared to eternal happiness rooted in truth, is not being burned to death and permanently excluded from reality a destiny deserving of terror?