The future's bright for executioners in 2016. The year began with another video nasty from so-called Islamic State (IS) featuring a new “Jihadi John” murdering alleged “spies” in customary orange jump suits.

This arrived 24 hours after the news that 47 prisoners had been beheaded, shot and/or crucified by our principal ally in the Middle East, Saudi Arabia. Among them was the Shi'ite cleric, Sheikh Nimr al Nimr, who had led a peaceful uprising against the Saudi monarchy.

The BBC's Nick Robinson put the question of the day to the former British ambassador, Sir John Jenkins yesterday: “What is the difference between Islamic State beheading its political opponents and Saudi Arabia beheading political opponents?” The former diplomat's reply made me choke on my porridge.

Saudi Arabia was “one of the community of nation states”, he said, and its actions were “disproportionate but understandable”. Understandable? Beheading, crucifying and shooting leaders of a rival Muslim faith is understandable, according to Sir John, because it is “within the law”, such as it applies in that oil-rich state.

It is not understandable, apparently, for IS to do the same in Syria because it is not the representatives of a legitimate state acting within the law of that country. Well, if that's how the Foreign Office thinks these days then hell mend us.

David Cameron has rightly condemned IS for its summary executions, but has remained silent about the actions of the Saudi beheaders. This is not only pusillanimous hypocrisy; it is also diplomatically inept.

We continue to regard Saudi Arabia, wrongly, as a force for peace in the Middle East. On the contrary: it is a force for war, and has aided the emergence of Sunni terror groups such as IS, al Qaeda and the al-Nusra Front. It did this by promoting the austere, sectarian Wahhabi form of Islam throughout the Middle East as part of its mediaeval conflict with the rival Shi'ite strand of Islam.

This Sunni/Shi'ite division dates back to the seventh century and a dispute over who was to succeed the Prophet Muhammad. It is similar to the division of the Christian faith into Protestant and Catholic to the extent that it is essentially sectarian today, with the doctrinal disputes lost in the mists of time.

Backing Saudi Arabia while condemning IS is like supporting the Soviet Union in the Cold War while condemning the activities of dictators like Nicolae Ceausescu in Romania; worse, it makes it look as if we support the Sunni side in this global Islamic schism. We should surely be distancing ourselves from both.

Saudi Arabia has spent billions of dollars on mosques, madrassas and arms to export its Wahhabi faith, which regards non-believers as enemies, and advocates beheading, flogging and crucifixion for crimes ranging from adultery and drug taking to disrespecting monarchy

Where do you think IS acquired their fondness for removing heads? It was from Saudi Arabia where no fewer than 157 people were executed in 2015 under Sharia law. The number of beheadings is rising as the country's wealth wilts because of the oil price collapse and the unelected Saudi princes fear an internal rebellion.

Saudi Arabia is a Frankenstein petro-monarchy the West backed originally as a source of hydrocarbons, then as a bulwark against communism and then as a counterbalance to the regional influence of Iran, a Shi'ite semi-dictatorship that executed even more people last year than Saudi Arabia. The Saudi princes are also a huge market for British and American arms manufacturers.

This monster is now beyond control. Yet Western leaders such as the Prime Minister still defer to the parasitic princes of Saudi Arabia because they can think of nothing better to do. They may be bastards, but they're our bastards, to paraphrase Lyndon B Johnson. Meanwhile, the Shi'ite/Sunni division spirals out of control and IS continues to mock Western intervention.

The chilling “letter to Cameron” read by the English-speaking, and evidently English-born, terrorist at the weekend was further confirmation that IS is a deranged death cult. But until we understand the equivalence between this kind of terrorism and the state terrorism of Saudi Arabia we will have no moral foundation for our latest war against terror.

But it is not just Britain and America who have double standards on the Saudi beheaders. The United Nations Human Rights body is chaired – you've guessed it – by Saudi Arabia. Perhaps it should have a sub-committee headed by IS.