Hassan Aboud’s practiced baritone belied the malevolence in his words.

“Oh Darraji!” he sang. “Our state provided us ammunition and sent us to assassinate you.”

That state is the self-proclaimed Islamic State, also known as ISIS or ISIL, the terrorist group that controls territory in Syria and Iraq and has recently projected violence to Ankara, Beirut, Paris and San Bernardino, California. A soft-spoken double-amputee sometimes carried to meetings by fellow gunmen, Aboud is an Islamic State commander who also directs a network of assassins, including those who killed Darraji, a former subordinate, with bullets and flame.

The recording of his singing circulated among past associates this year. A taunting dark requiem, it serves as evidence and confession. Aboud, who defected from Syria’s rebels to the terrorist group in 2014, was admitting to previously unsolved killings of former friends.

“We plucked Adeeb Abbas’ head,” he continued, naming another of his one-time deputies, blasted from a motorcycle by a roadside bomb. “We spilled his filthy blood.”

He then vowed to kill more, as a male chorus chanted to those marked to die: “We will liquidate every traitor.”

Since rising to prominence as an international menace, the Islamic State has tried to glorify its members, describing them as religious warriors who raised arms to protect fellow Sunni Muslims and serve their understanding of God. But the journey of Aboud, and his recruitment by ISIS, including with cash, departs from scripts emphasizing piety or civil defense.

It is the chronicle of an underground fighter maimed and darkened by his long fight, the biography — replete with rivalries and fratricide — of a proven and once popular Islamist commander whose actions turned more violent and vengeful as he moved into the Islamic State’s orbit.

Aboud, his former neighbors and associates say, abandoned the defense of his hometown for money, power and the license for viciousness that came with joining the Islamic State. His path resembled not the airbrushed arcs laid out in jihadi propaganda mills but a Middle Eastern mafia tale set against the corrupting effects of war.

The journey from jihadi rank-and-file to feared underground figure was shaped by multiple forces. These include the American invasion of Iraq in 2003 and the oppression of a border-straddling Sunni Muslim population by governments in Damascus and Baghdad. It was further stoked by the indiscriminate killing of civilians by Syrian security forces since 2011, then channeled by the patient plotting of a jihadi group, once shattered, that revived itself to eclipse al-Qaida.

Ultimately, his courtship by ISIS offers an unusually detailed look at how the group has selected commanders from a region that has produced uncountable militants since 2003. These chosen men, seduced by gifts and the Islamic State’s gloomy prestige, hold the terrain it needs to support its claim of being a caliphate.

Journalists for The New York Times met Aboud in Syria in 2013, as he led sieges around isolated army positions that were shelling the civilian population nearby; it was a fight he and the several hundred rebels he led, known as the Dawood Brigade, eventually won.

Sarmin, the town in the lowlands of Idlib province where he headquartered his brigade, was itself under frequent shelling, and he was moving and accepting meetings with much more caution than many rebel commanders.

The interview was arranged by the son-in-law of his boss, but Aboud’s supporters had everyone wait in the basement of a mosque for part of the afternoon before leading the group to an abandoned building in the partially evacuated, battle-scarred town.

Aboud was rushed in from the outside by those who carried him. He had lost fighters in battle the previous day and seemed weary and suspicious of guests. His gray pants had been folded to hide his lower-leg stumps, which he crossed in front of himself as he sat on a cushion. He opened the conversation in a quiet voice, and threatened to kill the journalists if he were misquoted.

Abu Ayman, a bomb-maker who helped carry him into the room, spoke more than Aboud, who chose his words with care, even when repeating Islamist boilerplate.

He complained about the activities of many secular rebels, describing them as opportunists and profiteers. “There are Free Syrian Army brigades,” Aboud said, that get weapons and “sell them in trade.” He asserted, again speaking softly, that Syria was being plunged into sectarian war by Iran and those it underwrites, including the Syrian government and Hezbollah. “Iran is seeking to re-establish the Persian Empire, to get control over the whole Middle East,” he said.

Immediately after the conversation he was picked up, carried swiftly outside and set on the front passenger seat of a muddy SUV, which sped away.

The Times returned to the Turkish-Syrian border after his defection and vendetta killings to interview those who worked with him closely.

Today Aboud, in his mid-30s, is an exile from Sarmin, where he had lived most of his adult life. Past associates refer to him as either an ISIS wali or emir, titles conveying authority or military power that the Islamic State bestows on governors and its middle rank.

They note that he did not simply drift to ISIS; he has had a relationship with the original underground Sunni insurgents in Iraq’s Anbar province, part of the crucible where the Islamic State formed, reaching back more than a decade.

Aboud and one of his brothers fought U.S. forces there in 2004 and 2005, several townspeople said. Some suggested that the pair returned to Syria as a sleeper cell tied to al-Qaida in Iraq, which was founded by Abu Musab al-Zarqawi and after his death in 2006 eventually became the Islamic State.

In the nearly year and a half since Aboud publicly joined the Islamic State, taking with him most of his fighters and many powerful weapons, he has been credited with, or blamed for, a sprawling mix of battlefield action and crime. Those who know him contend he led the capture of Palmyra, the town and ancient heritage site that ISIS defiled.

For all of Aboud’s activity, however, his story suggests limits to advancement within the group, which analysts say to a large degree remains led by Iraqis, including many connected to the dismantled Baathist regime of Saddam Hussein.

Hassan al-Dugheim, a rebel cleric who said he had observed Aboud since 2011, said his tactical skill and ruthlessness were beyond question. He bluntly added, however, that he considered Aboud stupid, and that the Islamic State had found in him a man who could be flattered, bought, then used.

“Syrians are for fighting,” he said, and those who had joined ISIS recently faced a glass ceiling inside. “They are like animals to be ridden, like a horse or a mule.”

Since Aboud arrived in Raqqa in 2014, his associates said, they have had only occasional insight into his militant life. ISIS, they note, is a such a closed system that little is known even of Abu Bakr al-Baghdadi, the Islamic State’s leader, and the nature of his power. Aboud and his brigade, once he defected, dialed back their social media presence and interviews. His activities, they said, have been assembled piecemeal.

One sighting of Aboud occurred in June in Palmyra. Khaled al-Homsi, an activist, said he was briefly imprisoned there, and Aboud toured the jail on crutches. “He met me privately in a room,” he said, “to convince me to pledge allegiance.”

After the Islamic State consolidated control in Palmyra, he added, some of the staff in the courts were from the Dawood Brigade, suggesting Aboud was trying to govern.

Efforts to reach Aboud this fall were unsuccessful. But former associates said they expected he would fail at such aspirations.

Ahmed al-Aasi, an activist with Ahrar al-Sham, a large Islamist fighting group, said Aboud’s role was obvious — as an enforcer, an instrument of purposeful violence to help ISIS gain territory and rule by fear. “With ISIS there are no limits, and you can abduct and kill whomever you like,” he said. “Hassan Aboud does not have a problem with killing. He likes it.”

The rebels’ former ally, he said, “is sick in his mind.”

A U.S. military official who analyzes the Islamic State said that under the pressure of airstrikes and internal strife, members with titles like emir and wali now gain rank through attrition, not design. “We watch the deck shuffle constantly, as they attempt to determine who will fit a role that has been vacated,” he said, “vacated” being a euphemism for “killed.”

Whatever Aboud’s eventual fate, a relative, who asked for anonymity so as not to draw his wrath, said, much of the legacy was already known. The recording of Aboud singing — coolly in tune as he described killing old friends — was a marker of a man lost to crime, a revolution soured and a people betrayed.

“His violence, his assassinations, his killing people — he is really behind this,” he said. “It is a mess now. Everything we have is a mess.”

© 2015 The New York Times Company