ISTANBUL—On the CCTV footage released by Turkish police, the widow of one of the Islamic fanatics responsible for last week’s terror rampage in Paris comes across as prim, even drab, as she goes through passport control at the airport here.

Hayat Boumeddiene’s tightly drawn white headscarf and hooded coat is a cultural world away from the scanty bikini she was wearing in a photograph that showed her on a beach fondly clutching future assassin Amedy Coulibaly. The holiday snap was taken before 2009, when she started to cover herself up with scarves and veils.

The transfer is startling from sun-worshipper and eager holidaymaker to the buttoned-up moll of an Islamic assassin. The 26-year-old looks giddily in love cuddling Coulibaly—a display of public affection hardly in keeping with the puritanical strictures of Salafi jihadis.

Her now-dead partner also used to pursue a lifestyle that clashed with the teachings of Islamic militants. Neither were paragons of religious rectitude. French police arrested Coulibaly on a string of theft and drug offenses before he embarked on the path of jihad and ended up gunning down four Jews at a kosher supermarket in Paris last week. In the caliphate of the self-styled Islamic State, where, according to Turkish authorities, Boumeddiene has found sanctuary and to whom Coulibaly apparently aligned himself, theft and drug use incur far worse punishments than those meted out by the unenlightened West—including flogging, amputation, and execution.

But then Boumeddiene and Coulibaly aren’t unique in having exited rowdy alternative lifestyles totally at variance with Islamic puritanism, embracing instead the simplicity of jihad. Although Coulibaly, it seems, observed the conservative demands a little less than his consort. During a 2010 interview with police investigators, Boumeddienne admitted Coulibaly “wasn’t really religious” and liked to “have fun.”

Some Westerners do indeed appear to have been devout before traveling to Syria or aligning themselves with jihadis—although how knowledgeable the really young ones or the obviously disturbed are about their religion remains questionable. Some of the frantic devotion has the ring of hollow religiosity, ritual without content, more cult-like than anything else.

Even so, Melanie Smith, a researcher with the International Centre for the Study of Radicalization, has argued that many of the estimated 200 or so Western girls and women who have gone to Syria to join the militants “tend to be extremely pious and have been IS fan-girls for the duration of the Syrian conflict.”

Aqsa Mahmood, a 20-year-old who was raised in a well-heeled Glasgow suburb and attended an exclusive Scottish girls’ school, fits into that profile. She led an orderly life as a teenager—wasn’t involved with boys, drugs or petty crimes. She seemed normal in most ways until she was lured and groomed online. And, according to her parents, she became more “concerned and upset” by reports of the Syrian conflict. “Aqsa, like many young people in our community, was naturally angry and frustrated at the loss of innocent life in the Middle East,” the parents said at a press conference last summer after their daughter ran off to Syria to become a jihadi bride.

Other recruits to the jihadist cause, though, appear to have had a more “secular” glide path, swapping what they see as the rootlessness and chaos of their lives for the false clarity and fake simplicity offered by al Qaeda or the Islamic State (also widely known as ISIS).

That appears to be more the explanation for the recruitment of Britain’s Sally Jones—an even more unlikely Salafi candidate than the bikini-wearing Boumeddiene. Jones was 45 years old when recruited and wasn’t even born into a Muslim or a minority immigrant family.

Now calling herself Sakinah Hussain or Umm Hussain al-Britani, Jones, a mom-of-two from the rural county of Kent in southeast England, sneaked into Syria in late 2013 after an online romance with Junaid Hussain, a young hacker-turned-militant from the English city of Birmingham. She is thought to be living in the town of Raqqa, the de facto capital in northern Syria of the Islamic State. In online exchanges with potential Western recruits, she claims to be enjoying the strict Sharia law of the caliphate, from whence she tweets blood-chilling threats.

Her most vicious micro-missive was in the wake of the mass decapitations of 50 Syrian soldiers, in which she declared: “You Christians all need beheading with a nice blunt knife and stuck on the railings at Raqqa... Come here I’ll do it for you!” She posts photos of herself posing with an AK-47 assault rifle and dressed in black niqab, which covers all of the face and body except the eyes. She and Hussain—he’s 25 years her junior—are now married.

But back in the 1990s she was a member of a smalltime girl punk rock band called Krunch and was then wielding a guitar rather than an automatic rifle. She was in and out of relationships and dead-end jobs. One video clip shows her wearing a low-cut top and tight leather mini-skirt. Neighbors in the town of Chatham have described her to British tabloids as a “nightmare”—an aggressive, anarchic woman who dabbled in witchcraft and drugs and threatened to put spells on them.

A purposeless, ungrounded life stands out with Boumeddiene, too. Born in the Paris suburb of Villiers-sur-Marne, she grew up in a rundown part of the town. Her mother was devout and died when Hayat was 6. Her father was unable to cope after his wife’s death and Hayat and some of her six siblings had to be taken into foster care. Her father visited her rarely and then appears to have broken with her after remarrying, although recently they are said to have reconciled. In care, she had to be moved frequently between foster homes because she proved troublesome and violent. She met Coulibaly in Juvisy-sur-Orge, southeast of Paris, while working as a cashier, a job she later lost because of her insistence on wearing the niqab.

One neighbor told French media that Coulibaly was the driving force in their partnership: “She left here with that man. He did everything and then it all came down on her. He was the mastermind.”

Maybe so, maybe not. The real masterminds seem to be their jihadi mentors, who knew how to channel the purposelessness and direct the anger. Of her religion, she told detectives in 2010, “It’s something which calms me down. I’ve had a difficult life and this religion has answered all my questions.”