Jamal Khashoggi entered the Saudi Arabian consulate in Istanbul on 2 October 2018, with the hope of finalizing a new marriage and a reasonable-enough sense of security. The Saudi dissident – a once well-connected loyalist turned prominent critic of the regime – had safely penned numerous laments of escalating censorship under Crown Prince Mohammed bin Salman as a columnist for the Washington Post. But awaiting Khashoggi, 59, was an assassination plot of gruesomely disproportionate scope: a hit-team of 15 Saudi officials who strangled the journalist, dismembered his body, and attempted to cover up the crime with a body double carrying his ill-fitting shoes out of the consulate.

The brazen hit operation and its horrifying details – 15 men sent to kill one journalist, a bone saw, the increasingly galling denials of Saudi involvement – caused international uproar, and tarnished the reputation of reform cultivated by Prince Mohammed. Khashoggi became shorthand evidence for the Saudi regime’s unrestrained impunity, and the feebleness of any meaningful international response. But for his friends who speak in Kingdom of Silence, a new Showtime film on the murder and its roots in the cozy relationship between the US and Saudi Arabia, Khashoggi’s killing was bizarre and surreal, a tangible hole. “Lost in all of this is that he was just a great guy, and he had a great sense of humor,” Lawrence Wright, a journalist for the New Yorker and longtime friend of Khashoggi, told the Guardian.

Kingdom of Silence is part retelling of Khashoggi’s personal and philosophical evolution as one of the Arab world’s most prominent journalists, and part history of the strange, toxic relationship between the US and Saudi Arabia – a taproot of influence, power and greed underpinning Khashoggi’s life and work for decades, and the impunity behind his assassination.

Khashoggi, born in Medina in 1958, was “a guy who lived his life at the center of a whirlwind”, Rick Rowley, the film’s director, told the Guardian. As a young adult in the 1980s, Khashoggi embedded with the mujahideen, a band of Islamist, Saudi-backed rebels, warring against Soviet forces in Afghanistan, and befriended their charismatic, elusive leader, a Saudi construction heir named Osama bin Laden. (Khashoggi was the first journalist to capture the man who would later orchestrate the 9/11 attacks on camera.) After the war, Khashoggi “came back [to Saudi Arabia] with a reputation”, said Wright, as a connected reporter in a country with an entirely state-owned press.

Kingdom of Silence traces Khashoggi’s evolution from a man sympathetic to the mujahideen to a critic heartbroken over the terroristic turn of his former sources. “Time and again,” said Rowley, “he saw his heroes – heroes he helped create, whose stories he told, who he championed and defended – he saw them turn into villains, and do things that he recognized as evil. And to his great credit, he allowed himself to be wounded by that, and was changed by it.”

By the time Wright met him in 2003, Khashoggi was a “friendly critic” of his home country, acknowledging the gap between the vision of Saudi Arabia and the real one. Khashoggi was initially a supporter of the American invasion of Iraq – a power-play only possible with Saudi support – and worked for years as an aide to Prince Turki bin Faisal al Saud, the head of Saudi intelligence. But he supported the revolutions of the Arab Spring, and was crushed by Saudi Arabia’s role in snuffing out the democratically elected president of Egypt in favor of a military coup.

By 2017, Mohammed bin Salman had consolidated power in Saudi Arabia, and instituted some reforms – allowing women to drive, permitting cinemas – while simultaneously cracking down on reformers. Feeling increasingly threatened, Khashoggi left the country, his family and his career, shortly before the crown prince detained 30 members of the royal family, including Khashoggi’s former patron, in the Riyadh Ritz-Carlton in what was essentially a power grab.

As an exile, Khashoggi “was determined to expand the zone of free speech”, Wright said. The last time they spoke, at an event Wright hosted in Austin, Texas, Khashoggi lamented crackdowns on free speech in his home country, even as the crown prince cultivated an image of reform in a tightly choreographed goodwill tour with US tech executives and VIPs in the spring of 2018.

For a country so concerned with international reputation, why risk so much goodwill on the heinous murder of one journalist? Kingdom of Silence explores the risk Khashoggi seemingly posed to the regime, both personal and financial, to trigger such an outsized, staggeringly violent response. It wasn’t just Khashoggi’s words in the Washington Post; as an aide to the head of Saudi intelligence, Khashoggi had facilitated meetings with Islamists and Saudi intelligence, and knew “more about Saudi support for Bin Laden and al-Qaida early on, and whether it continued and who it continued with, than anyone else on the planet”, said Rowley. And in the days before his murder, Khashoggi had agreed to meet with the lawyers of 9/11 families suing Saudi Arabia for the country’s role in the 2001 attacks.

There was also the Saudi regime’s intention to burnish a Putin-esque reputation of power and fear. “The conceit in Jamal’s assassination was ‘they won’t be able to pin it on us, but they’ll know,’” said Wright. This only failed in part; an investigation by the CIA found Mohammed bin Salman ordered the killing, despite the official word from Riyadh that the 15 close associates constituted a “rogue operation”. But the pin is blunt – international recrimination aside, there’s no sanction, no consequence, no recourse. “Authoritarian thugs around the world will see that there’s license to commit murder,” said Wright.

The kingdom has conducted what humanitarian experts have called a parody of justice: eight unidentified men were charged, though Khashoggi’s sons, who remain in Saudi Arabia and thus under threat of coercion, said in May that they had “pardoned” the killers. The hushed trial, which Khashoggi’s fiancee Hatice Cengiz has called “a complete mockery of justice”, solidified that “there’s total impunity to get away with everything”, said Rowley.

But Kingdom of Silence resists stripping the brutality of Khashoggi’s killing from its geopolitical context, and who that implicates. “This crime isn’t separate from us,” said Rowley. “It’s easy to say, ‘Oh look at the barbaric regime thousands of miles away in the desert, they’re nothing like us. How could such evil exist anywhere in the world?” But Saudi power is undergirded by longstanding US support, itself bolstered by oil and weapons deals. “This is an evil of our own making,” said Rowley.

Though the Senate has voted to block billions of dollars in arms sales to the kingdom and to cut off military support for Saudi Arabia’s devastating war in Yemen, Donald Trump, consistently a fan of authoritarian leaders, has vetoed both measures and instead approved an $8.1bn arms sale. The consistent support from the US has “created an atmosphere of impunity in which a crime like this was possible and imaginable”, said Rowley.

Kingdom of Silence implicitly argues for large-scale change – a recalibration of the US-Saudi relationship, the end to the war in Yemen – that seem unlikely given the current political reality. But there’s justice to be found on a smaller level, Rowley said. Khashoggi “was murdered to silence him”, and reading Khashoggi’s words now, his many columns on Prince Mohammed’s crackdowns and unbearable oppression in his former home, would be “a kind of victory over silence they would impose”.

“How much of a better world would it be if someone like Jamal has the opportunity to speak?” wondered Wright. “His critiques would be so helpful to guiding that nation toward a more democratic and prosperous future. Without that voice, there’s a vacancy. And in that vacancy, people will live in fear, and justice won’t be served.”

• Kingdom of Silence premieres on Showtime on 2 October with a UK date to be announced