A lot of people fear hell when they're leaving religion, but by studying the history of hell, that fear can be reduced. Islam took ideas about hell from other local religions, including the Zoroastrian, Christians and to a lesser extent, Jews, and the Arabic word Jahannam is directly transliterated from the Hebrew Geh-Hinnom.
Geh-Hinnom is a real place that exists, but it isn't anything like its descriptions in Muslim texts, and therefore the people who wrote those texts probably didn't know anything about the real Geh-Hinnom and were just copying and pasting directly from other texts, and just adding embellishments to existing myths. Geh-Hinnom means "valley of Hinnom", and it is located outside what would have been, in ancient times, the walls protecting Jerusalem. Hinnom was a guy who is purported to have owned the land millennia ago, although to my knowledge, there is no real evidence that he existed.
The badness of Geh-Hinnom was in its being a burning landfill. It was used as a landfill because of its convenient location outside the city, but without a good way to bury the trash to keep it from smelling, it was set on fire. This was before electronics were invented and metal would have been too precious to throw away, so the stuff in the trash would have been organic in nature, easily flammable without expelling toxic fumes. The thing that originally made it seem like a threat as a place of repose after death is that no one wants to have their dead body thrown in a dumpster and set on fire; this wasn't meant to be a threat of eternal judgement at first, just a really unpleasant thing that could happen if you were a public nuisance.
It was especially strong as a threat when you consider that there was, at least on some level, ancestor worship and a belief that the dead were in some way alive in their graves and able to effect the living. This is especially evident in the things they wrote about their patriarchs, people like Samuel, David, and Abraham. They believed that these people were not fully dead, they were "resting with their fathers", and able to, by acts of their own will, effect the world of the living. They could be consulted and could curse or bless you, leading to good or evil happening in your life.
The earliest texts to mention Geh-Hinnom as a place of repose after death do not consider it eternal. They state that the maximum amount of time a person can stay there is about a year. To them, this was probably not about the soul in the modern sense of the word; it was probably about how quickly the body would decompose sufficiently to leave the landfill completely and return to the cycle of nature. They probably meant that within a year, the body would be absorbed back into the ground or life cycle of nature, freeing the ancestor spirit associated with it from any suffering it may be experiencing in its living death. But it was not a place of torment of an eternal soul, at least not in the same way that Plato would have thought of the soul.
A legend began circulating at some point that there was, at one time, child sacrifice that Hinnom or his sons allowed to happen on their land, which was why it was used as a landfill by later peoples. This is more than likely a legend, for several reasons. First, there is no archaeological evidence of this ever happening, either in the valley or in the nearby areas. Second, the accounts of this were written by enemies of the people who are purported to have done it; enemies of people are, in general, a bad place to go to get information about the people in question, as stories tend to be exaggerated and sometimes even made up, even in the modern era (such as weapons of mass destruction purported to be in places where they weren't). Third, the accounts don't appear to be from the same time as the events they describe; they seem to come centuries later, increasing the probability that it wasn't true.
The idea of Geh-Hinnom continued to metamorphose. The epistle of James, possibly written before the canonical gospels, mentions Gehenna (Greek for Geh-Hinnom) as a place from which evil comes. By the time the Christian gospel writers were putting down their ideas, they wanted to make clear that their ideas of Gehenna were different. The earliest of the canonical gospels, Mark, definitely believes that the punishment is eternal:
Mark9:47 "If your eye causes you to stumble, throw it out; it is better for you to enter the kingdom of God with one eye, than, having two eyes, to be cast into geh-hinnom 48 where their worm does not die, and their fire is not quenched." Matthew and Luke, which both probably used Mark as a source material in their works, refer to Gehenna as well. Matthew promises doom of Gehenna liberally. Luke only refers to it once, warning that God has the authority to throw you there, and so you should be careful not to cross him.
Christian ideas changed in the next few centuries, and soon had adopted a very Platonist view of body/mind duality and the separation of an ethereal soul from a physical body. This is a topic that can be covered at length, and I'm not going to really get into it, but the idea is that the body is a part of the physical world, the physical world is evil, and only by shunning the physical world can the ethereal soul be freed to enter a more pure realm.
Islam incorporated many of the ideas of the version of Christianity local to its thinkers. It also enveloped the Persian empire within a hundred years of being founded and incorporated ideas from Zoroastrianism, such as the resurrection of the dead before a final judgment of individual souls, each standing and accounting before God, and of the sun and moon darkening before the final judgment; as well as ideas about hell specifically, such as that hell is full of foul smells and vile food, and souls being tightly packed together but each believing themselves to be in total isolation. What is most interesting about this is that the conquest of the ancient Persian empire (completed by the end of the reign of Umar, the second caliph) occurred prior to the commitment to writing of any of the Islamic texts, including the Quran (begun during the reign of the third caliph, Uthman). Also from Zoroastrianism came the idea that the punishments of hell fit the crimes of the punished, and that there were individualized punishments for individual punishments for individual crimes. This idea found its way into the Christian world via Dante’s Divine Comedy and in particular Inferno, written between 1308-1320, although it does not appear within the Christian doctrinal texts. Dante more than likely got the ideas from Islam, which in turn got them from Zoroastrianism; but Zoroastrianism is a non-Abrahamic faith, and is not considered to be part of the “Judeo-Christian tradition”, whatever that phrase means.
But what about Jahannam? It still exists as a place in the real world, and none of these tortures are happening there. Today the landfill is gone and there's no fire, and there are no eternal worms. One can go to Jahannam and see that none of the insane tortures from the Muslim texts are happening there. No one is being hung from their hair or flayed alive. The fire that was deemed unquenchable by the Christian texts was put out. There are no “living dead” ancestors walking around cursing you. So apostates shouldn't be afraid of jahannam when they're told by their friends and family that that is their future destination, because it's really not all that bad. In fact, there's even a water park and a swimming pool.
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