Saudi Arabia is facing its biggest diplomatic crisis with the west since the 9/11 attacks on the Twin Towers – and it is one almost certainly made by royal hands. The reason is that the disappearance of the Washington Post columnist Jamal Khashoggi looks directly traceable to the government in Riyadh. The evidence points to Mr Khashoggi being tortured, killed and mutilated after walking into the Saudi consulate in Istanbul by a hit squad who had been sent on the orders of the kingdom’s crown prince Mohammed bin Salman. Riyadh is preparing to say Mr Khashoggi was killed by mistake during an interrogation. But if it turns out that the crown prince signed off on a plan to abduct or even kill a critic in Istanbul then it raises his despotism to the level of tyrants such as Saddam Hussein and Muammar Gaddafi, who used their embassies to terrorise exiles.

Prince Mohammed has been the power behind the throne in Saudi Arabia ever since his father King Salman became the country’s absolute ruler in January 2015. While pledging to modernise the conservative kingdom, the crown prince has locked up human rights activists and clerical critics. This month a Saudi economist who disparaged the flotation of the Saudi state-owned Aramco oil company, a key plank of Riyadh’s economic strategy, was charged with terrorism. The crown prince is the hardliner in the Gulf row with Qatar and the architect of the disastrous war in neighbouring Yemen, where Saudi forces have bombed and deliberately starved the Middle East’s poorest nation. He cares little for universal rights or the liberal international order and has been emboldened by his friendship with Donald Trump, whose first foreign trip as US president was to Riyadh. It is Mr Trump who appears to be giving cover to the ludicrous idea that a 15-man torture team – which included a forensics doctor – killed Mr Khashoggi but only because they got carried away.

Until now, western governments and businesses had mostly been willing to ignore human rights issues. But Mr Khashoggi’s disappearance signalled that even carefully argued criticism about growing repression in Saudi Arabia and the calamity in Yemen was seen as sufficient excuse for murder. Mr Khashoggi’s body, say the Turks, was carved up with a bone saw and smuggled out in a black Mercedes van. No wonder executives are pulling out of the kingdom’s annual “Davos in the Desert” shindig. Investors have also fled the Saudi stock market. The crown prince’s sobriquet MbS now sees him darkly mocked as Mr Bone Saw. In Britain and America leading lawmakers say it may be time for sanctions. In response the Saudis have threatened to weaponise their vast oil reserves and buy arms from Moscow.

The reigns of Saudi kings in recent decades have been dominated by health concerns rather than navigating foreign and domestic turbulence. Leaders can lose their minds in office. But rarely do they gain top positions when they have already lost the plot. King Salman ascended to the top job when rumours that he was suffering from dementia were rife. That is what makes Prince Mohammed’s position so concerning. This latest episode ought to focus minds in Riyadh on the suitability of the crown prince for the top job. He is officially the heir to the throne. Only in his 30s, Mohammed bin Salman could rule for a long time – a future that would be marked by instability if the past few years are any guide. King Salman has other talented sons. There are also other gifted royals. Prince Mohammed is King Salman’s third crown prince. If he had any hand in the events of the past fortnight, it must be time for the House of Saud to find a fourth.