Patrick Karegeya knew Paul Kagame well. The pair went to school together, worked alongside each other in Ugandan intelligence and then fought to free their country from the genocidal gangsters who unleashed horror in their native Rwanda. When Kagame became president, Karegeya was put in charge of foreign intelligence services.

But after a decade, their disagreements, including over human rights and attacks on neighbouring Democratic Republic of Congo, became too strong. He was relieved of his duties, stripped of his rank as colonel and jailed. Once free he fled, later joining forces with three other prominent exiles to lead opposition to Kagame’s government.

Knowing the Rwandan president so well, Karegeya was under no illusions what might happen to him, especially after his friend Faustin Kayumba Nyamwasa was shot in the stomach in South Africa in 2010. “The Rwandan government can no longer tolerate any dissent,” he said last year. “There is a deliberate plan to finish us off.”

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Now the plain-speaking Karegeya is dead, his brutalised body discovered in the room of a luxury South African hotel. A murder investigation has been launched. It seems he was strangled, a rope from the hotel curtains found with a bloodied towel in the safe.

Rwandan officials deny any complicity. They always do, of course. It is part of the regime’s tactics, their smart diplomats throwing up smokescreens while smearing enemies and exploiting global sympathy for the genocide.

But Nyamwasa, a former Rwandan army chief who has survived two assassination attempts, asked who else might want to kill his friend. “It is not the first time and it is not the last. Most of President Kagame’s political opposition are in exile or in prison or are dead.”

It may take time for the full facts to filter out. Initial reports say police want to interview a Rwandan man who met Karegeya at a rail station then went with him to the hotel in the upmarket suburb of Sandton.

Yet one thing is certain beyond the death of an important dissident. Enemies of Kagame – the despot so beloved by Western democratic leaders and charity dupes – seem to have a strange habit of dying in disturbing circumstances.

Over the years a succession of prominent critics and campaigners, judges and journalists, have been killed. They have been beaten, beheaded, shot and stabbed, both at home in Rwanda and abroad in nervous exile. Some were good people, others far from saints – and their deaths came after crossing Kagame.

“We don’t know the details of how and why Karegeya was murdered but there is a long established pattern of assassination and attempted assassination of Rwandan government critics,” said Carina Tertsakian, senior researcher on Rwanda at Human Rights Watch.

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Kagame’s strategy has been clear from the start of his rise to power; indeed, defectors and dissidents have explained in detail how he man gets rid of his rivals. “He believes that all opponents must die,” said Karegeya last year.

Those who served as his aides, army officers and bodyguards have said that even in exile during the days of bush warfare, he eliminated those who threatened his authority. After taking power following the 1994 genocide, his repressive regime used murder, arbitrary arrest, jail and strict media controls to sustain its incredibly rigid rule.

Former colleagues told me he never hid what would happen to enemies; even Paul Rusesabagina, the hotel manager who became a global hero amid the hell of genocide, had to go into hiding.

All too typical was the story of Seth Sendashonga, the respected Minister of the Interior in the post-genocide government. After protesting human rights abuses in a series of memoranda sent to Kagame, he was dismissed and went into exile in Kenya, where he became increasingly vocal against the government.

After surviving a first assassination ambush in February 1996, in which an arrested man with a firearm turned out to be an employee at the Rwandan embassy, he was shot dead in Nairobi two years later. The case bears similarities to the recent attacks in South Africa.

This killing of critics has happened with relentless regularity. There was a particularly nasty spate before the 2010 election, when not only was Nyamwasa targeted but a newspaper editor murdered, a rival politician found near-beheaded and even a Tanzanian law professor involved in a genocide case shot dead.

The following year Scotland Yard warned two exiles in Britain that a Rwandan hit squad had been sent to kill them, although they were not high-profile. Scandalously, even this did not stop the flow of British aid and adulation.

One of the targets was Rene Mugenzi, a father of three and Liberal Democrat activist. He had to cut off contact with many fellow Rwandan exiles in Britain for fear they might be government agents and still lives under a high state of security alert.

“This latest case is very troubling for me and my family,” he told me. “You just feel anything can happen, especially when nothing is done at the international level against Kagame. It is like he has a licence to kill.”

And this is the key point. For despite the murders, the abuse of human rights, the locking up of political rivals, the ceaseless and now well-documented stoking of carnage and conflict in the Congo, Kagame remains a leader lionised in Washington and Westminster.

The world’s foremost scholar on Rwanda has described him as “probably the worst war criminal in office today.” Another leading academic concluded he was running “a very well-managed ethnic, social and economic dictatorship”.

But Bill Clinton calls him “one of the greatest leaders of our time” while Tony Blair, who works closely with him and has borrowed his plush private jet, hails him as “a visionary leader”. There is similar adoration on the right among many Tories and Republicans; Rwanda was even welcomed into the Commonwealth four years ago.

This disgusting hypocrisy, fuelled by the desperate search for an aid success story, is underlined by Kagame’s intelligence chief meeting ministers in London despite being indicted by a Spanish judge, while Theogene Rudasingwa, a leading Kagame opponent based in the United States, is refused a visa.

Rudasingwa, Kagame’s former chief of staff and one of his key opponents alongside Karegeya, is dismayed by Western reluctance to acknowledge Kagame’s criminality despite a welter of evidence.

So was he scared following the latest apparent murder, I asked him on Friday? “No,” he replied. “This just makes me more determined. I know he is on a mission to kill all of us but we are going to fight him to the finishing line.”

These are brave words, given what has happened to so many of those who challenged Kagame. Yet Britain, to our lasting shame, continues to back the monstrous killer in Kigali.