BURAIDA, Saudi Arabia — The men were not hardened militants. One was a pharmacist, another a heating and cooling technician. One was a high school student.

They were six cousins, all living in Saudi Arabia, all with the same secret. They had vowed allegiance to the Islamic State — and they planned to kill another cousin, a sergeant in the kingdom’s counterterrorism force.

And that is what they did. In February, the group abducted Sgt. Bader al-Rashidi, dragged him to the side of a road south of this central Saudi city, and shot and killed him. With video rolling, they condemned the royal family, saying it had forsaken Islam.

Then they fled into the desert. The video spread rapidly across the kingdom, shocking a nation struggling to contain a terrorist movement seen as especially dangerous not just because it promotes violence, but also because it has adopted elements of Saudi Arabia’s conservative version of Islam — a Sunni creed known as Wahhabism — and used them to delegitimize the monarchy.

“Wahhabism is fundamental to the Islamic State’s ideology,” said Cole Bunzel, a scholar of Wahhabi history at Princeton University and the author of a recent paper on Saudi Arabia and the Islamic State. “It informs the character of their religion and is the most on-display feature, in my opinion, of their entire ideology.”

Among 20 terrorist episodes in Saudi Arabia since late 2014, the killing of Sergeant Rashidi was the third in which citizens had secretly joined the Islamic State and killed relatives in the security services. In each case, they justified their acts by saying Saudi Arabia practiced a corrupted version of the faith, a charge aimed at a kingdom that holds itself up as the only true Islamic state.

The Islamic State, like Al Qaeda before it, accuses the Saudi monarchy of corrupting the faith in order to preserve its power. But Qaeda networks in the kingdom were dismantled years ago, and the group’s leadership abroad has discouraged killing Muslim civilians.

The Islamic State, however, has been able to infiltrate the kingdom through digital recruiting, and it has found devotees willing to kill fellow Sunnis, as well as Shiites, to destabilize the monarchy.

In July, a 19-year-old man murdered his uncle, a police colonel, before carrying out a suicide attack near a prison, wounding two guards.

In an audio message released by the Islamic State after his death, he addressed his own mother.

“Your apostate brother was a loyalist to the tyrants,” he said. “Were it not for him, the tyrants would not exist.”

Maj. Gen. Mansour Turki, a spokesman for the Saudi Interior Ministry, said that terrorist attacks over the past two years had killed scores of people, along with about two dozen militants.

In addition, about 3,000 Saudis have joined militant groups abroad, and more than 5,000 have been incarcerated at home on terrorism charges, a large increase in recent years.

Saudi Arabia has a tangled history with Islamic militant groups. For a long time, it backed them as proxy forces to push its agenda in places like Bosnia, Chechnya and Afghanistan (where it worked with the United States). But that largely ended in 2003, when Al Qaeda turned its focus on the kingdom and staged a series of deadly attacks.

Now the Islamic State poses a new challenge, by turning aspects of Saudi Arabia’s conservative creed against it. Wahhabism has been molded over the years to serve the interests of the monarchy, emphasizing obedience to the rulers and condemning terrorist attacks, even against those seen as apostates.

Still, among the Islamic State’s many enemies, Saudi Arabia is the only one that considers the Quran and other religious texts its constitution, criminalizes apostasy and bans all forms of unsanctioned public religion.

The country was founded on an alliance between the Saud family, whose members became the monarchs, and a cleric named Sheikh Muhammad ibn Abdul-Wahhab, whose teachings were used to justify military conquest by labeling it jihad against those deemed to be infidels, most of whom were other Muslims.

Sheikh Abdul-Wahhab’s descendants still dominate the religious institutions of the Saudi state, which now play down the violence in the country’s history and emphasize aspects convenient to an all-powerful royal family, like the importance of obeying the leadership.

Saudi officials reject comparisons between their ideology and that of the Islamic State, noting that millions of non-Muslims live in the kingdom and that the government is closely allied with the United States and participates in the American campaign against the militant group.

They also say that Saudi Islam does not promote the caliphate, as does the Islamic State, and that senior clerics condemn the terrorist attacks and have branded the group “deviant.”

But critics argue that many Saudi clerics have never renounced the aspects of the Wahhabi tradition that the Islamic State has adopted, especially with regard to Shiites, who make up an estimated 10 percent of the kingdom’s 20 million citizens. Many Saudi clerics consider Shiites heretics and accuse them of loyalty to Saudi Arabia’s regional rival, Iran.

The jihadists have exploited this by repeatedly launching suicide attacks on Shiite mosques and then accusing Saudi clerics of hypocrisy for condemning the violence.

“It is clearly hard for Saudi clerics to condemn outright attacks on Shiites,” said Mr. Bunzel, the Princeton scholar. “And you get the feeling that they don’t care as much if the Shiites get attacked, since they’re not really Muslims in their view.”

As elsewhere in the world, the Islamic State has relied on social media to reach inside the kingdom, find recruits and dispatch them to attack, often under the noses of their closest relatives.

This has made plots hard to prevent, General Turki said, citing the example of a man arrested last year after killing two police officers in a drive-by shooting near Riyadh. One Islamic State supporter had given him the car, and another had provided the gun, but the attacker never learned their names.

Still, the group had struggled to target the security forces, so it told recruits to kill officers from their own families. General Turki summarized their message as, “You are closer, so no one will know you.”

In September, two men abducted their cousin, a soldier in the Saudi Army, and filmed a video of him bound and begging for mercy in the sand as they pledged allegiance to the Islamic State and shot him dead. The security forces killed one of them and arrested the other.

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Soon after came the abduction and murder of Sergeant Rashidi by the six cousins, following a process of radicalization that no one in their families noticed.

The ringleader, Wael al-Rashidi, who addressed the camera in the video, had a pharmacy degree and worked in a Riyadh hospital, two of Sergeant Rashidi’s brothers said in an interview. He had smoked cigarettes, a practice shunned by most devout Muslims, and spent hours “hacking” on his computer or playing war games on his Xbox.

One of his two brothers who joined the plot played the oud, also an act religious conservatives frown on.

Three other attackers were university students; two were brothers, and two were roommates, both studying Shariah at a state university in Riyadh.

About two years ago, Wael al-Rashidi became more religious and withdrew from family functions, but that did not raise alarms in a society where religious conservatism is common.

“We all saw that he was growing a beard and going to the mosque, but there are lots of people like that here, so we had no idea he was planning something else,” said Sergeant Rashidi’s foster brother, Mishari.

Even more puzzling was that Wael’s father was a retired officer in the domestic intelligence service, an agency charged with detecting jihadist threats, Sergeant Rashidi’s brothers said. The father of another of the attackers still works for the force.

“Maybe he didn’t know, or he was scared to report him or thought he would straighten out,” said one of Sergeant Rashidi’s brothers, Bandar. “Only he knows.”

While the attackers were not personally close to Sergeant Rashidi, they told him they had a gift from their mother for him to deliver. He met them, and they abducted and killed him a few hours later.

The family’s dread grew in the following days as relatives noticed that their sons were missing and had turned off their phones. Then the Islamic State released the video, confirming the family’s suspicions.

On March 11, Saudi security forces tracked the six men to a remote location and killed them all in a shootout, local news outlets reported.

Sergeant Rashidi’s brothers are glad the six are dead, but the episode has left them deeply suspicious.

“If anyone calls and says, ‘Where are you? I want to see you,’ you don’t trust him anymore,” Bandar said.