Peter R. Neumann is professor of Security Studies and director of the International Centre for the Study of Radicalization (ICSR) at King's College London. The ICSR released a new report Monday, "The Narratives of Islamic State Defectors," cataloging the experiences of those who leave ISIS.

London (CNN) Much has been written about the young men and women who join the Islamic State. We are familiar with their biographies and pathways, backgrounds and motivations.

But virtually nothing is known about those who quit: the "defectors" who didn't like what they saw, abandoned their comrades and fled the Islamic State. Yet their stories could be key to stopping the flow of foreign fighters, countering the group's propaganda and exposing its lies and hypocrisy.

For a short paper , I collected all published stories about people who have left the Islamic State and spoken about their defection. I discovered a total of 58 -- a sizable number but probably only a fraction of those who are disillusioned or ready to leave.

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They are a new and growing phenomenon. Of the 58 cases, nearly two thirds of the defections took place in the year 2015. One third happened during the summer months alone.

The defectors' experiences are diverse. Not everyone has become a fervent supporter of liberal democracy. Some may, in fact, have committed crimes. They were all, at some point, enthusiastic supporters of the most violent and viciously totalitarian organization of our age. Yet they are now its worst enemies.

The quality of their testimony varies, and the precise circumstances and reasons for leaving the Islamic State aren't always clear. What convinced me that, as a whole, their stories are credible is how consistent their messages were.

Among the 58 defector stories, I found four narratives that were particularly strong:

One of the most persistent criticisms was the extent to which the group is fighting against other Sunni rebels. According to the defectors, toppling the Assad regime didn't seem to be a priority, and little was done to help the (Sunni) Muslims who were targeted by it.

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Most of the group's attention, they said, was consumed by quarrels with other rebels and the leadership's obsession with "spies" and "traitors." This was not the kind of jihad they had come to Syria and Iraq to fight.

Another narrative dealt with the group's brutality. Many complained about atrocities and the killing of innocent civilians. They talked about the random killing of hostages, the systematic mistreatment of villagers and the execution of fighters by their own commanders.

None of the episodes they mentioned involved minorities, however. Brutality didn't seem to be a universal concern: it was seen through a sectarian lens, and caused outrage mostly when its victims were other Sunnis.

The third narrative was corruption. Though none believed that corruption was systemic, many disapproved of the conduct of individual commanders and "emirs." Syrian defectors criticized the privileges that were given to foreigners, for which they claimed was no justification based on the group's philosophy or Islam in general.

While many were willing to tolerate the hardships of war, they found it impossible to accept instances of unfairness, inequality and racism. "This is not a holy war," said a defector from India, whom the group had forced to clean toilets because of his color of skin.

A fourth narrative was that life under the Islamic State was harsh and disappointing. The defectors who expressed this view were typically the ones who had joined the group for "selfish" reasons -- and who quickly realized that none of the luxury goods and cars that they had been promised would materialize.

For others, their experience in combat didn't live up to their expectations of action and heroism. One of them referred to his duties as "dull" and complained about the lack of deployments, while another claimed that foreign fighters were "exploited" and used as cannon fodder.

These stories matter. The defectors' very existence shatters the image of unity and determination that the group seeks to convey. Their narratives highlight the group's contradictions and hypocrisies. Their example may encourage others to follow, and their credibility can help deter wannabes from joining.

In my view, governments and civil society should recognize the defectors' value and make it easier for them to speak out. Where possible, governments should assist them in resettlement and ensure their safety. They also need to remove legal disincentives that prevent them from going public.

Not every defector is a saint, and not all of them are ready or willing to stand in the public spotlight. But their voices are strong and clear: "The Islamic State is not protecting Muslims. It is killing them." They need to be heard.