Gen. Emmanuel Karenzi Karake, the head of the Rwandan intelligence services who is now free on bail in London pending an extradition hearing in October, is an exemplar of the tightly knit group of diaspora Tutsis that ousted Rwanda's Hutu-dominated government and rose to power during the 1994 genocide. Once a victim of human rights abuses, he now stands accused as a perpetrator. He is also a hostage — not of any justice system, but of an increasingly lone and erratic dictator, President Paul Kagame, who uses and abuses his inner circle as he grows more desperate to stay in power.

General Karake was arrested on June 20 at Heathrow airport on charges brought by a Spanish investigative judge, who accuses him of crimes against humanity and war crimes committed between 1990 and 2002 in Rwanda and the Democratic Republic of Congo. In particular, he is suspected of ordering the killing, in 1997, of three Spanish nationals working for the relief organization Medicos del mundo. 

The fact that General Karake was arrested in Britain, a country he has visited with impunity for years, and hitherto one of post-genocide Rwanda's staunchest allies, suggests that Mr. Kagame himself is no longer beyond the reach of international justice. It also shows that the fate of the two men, so close for many years, is inextricably linked. General Karake has nothing to gain from a break with Mr. Kagame, beyond a stiff sentence from a foreign tribunal. But the reverse is equally true. Both men can only be free and powerful in Rwanda.

The arrest was only the latest twist in the wandering life of Mr. Kagame's spy chief. Both men were raised in Uganda, where their parents sought refuge after a revolution in 1959 led to the creation of a Rwandan "Hutu Republic." The young Mr. Karake earned degrees in business and international studies from universities in Uganda and Kenya, and an M.B.A. from the University of London before joining the Rwandan Patriotic Front, fighting to reclaim power in Kigali. The civil war, started in 1990 by the RPF, culminated in the massacre by Hutu extremists in 1994 of an estimated 800,000 civilians — mainly Tutsis but also Hutus who opposed the killing.

After the rebels took Kigali, Mr. Karake became a member of Mr. Kagame's inner circle — mostly English-speaking Tutsi returnees from Uganda — that now rules Rwanda. But even as the new government won the favor of a Western leadership riddled with guilt for failing to halt the genocide, Mr. Kagame and his men orchestrated revenge killings. During the second half of 1994, at least 40,000 Hutus were killed inside Rwanda. Between October 1996 and May 1997, another 200,000 perished as they fled across Zaire, now the Democratic Republic of the Congo. Many died of hunger or disease or at the hands of RPF fighters and their Congolese rebel allies, whom a U.N. investigation accuses of crimes against humanity and possibly "acts of genocide."

In 2007, General Karake was appointed deputy commander of the African Union peacekeeping force in the Sudanese province of Darfur. Though he was promoted deputy commander of the larger U.N. mission in Darfur in January 2008, the United Nations was embarrassed by allegations by Human Rights Watch that forces under his command had massacred Congolese civilians in the battle for Kisangani in 2000. Nevertheless, the U.N. extended the general's tenure beyond its expiry in October 2008 after Mr. Kagame threatened to pull the Rwandan contingent out of Darfur. Both the United States and Britain, which have praised Mr. Kagame for his efforts to lift Rwanda out of atavistic poverty, also exerted pressure on the United Nations.

Since then, in the face of an increasingly vindictive and megalomaniac leader, the phalanx controlling Rwanda has broken up. Many of Mr. Kagame's closest supporters have opposed the president's iron-fisted methods on the grounds that they jeopardize not only their own future but the future of the minority Rwandan community. Rebuked and punished by Mr. Kagame, several of his former associates have fled into exile. Some have been assassinated, including one of General Karake's predecessors, Col. Patrick Karegeya, who was murdered in a Johannesburg hotel room on Jan. 1, 2014. General Karake fell from grace in April 2010 when he was arrested for "immoral conduct." Like many colleagues, he was cashiered and detained, but eventually rehabilitated after he was made to understand that his destiny is entangled with that of his boss.

Mr. Kagame can't leave office without risking arraignment by the International Criminal Court, a threat from which even a trusted successor could not shield him — and Mr. Kagame no longer trusts anyone. Ignoring constitutional limits, he is orchestrating a "popular" movement to seek another term in 2017. Another election "victory" lies ahead, unless justice catches up with him. Thus the president is defending General Karake as if he himself were being forced to remain in London. He has denounced his spy chief's arrest as a continuation of "colonialism" and accused the British of "arrogance and contempt" for complying with what is in fact a legal obligation.

Mr. Kagame has also called on his foreign allies for support. Cherie Booth, former Prime Minister Tony Blair's wife, leads the Karake defense team. Two of the Rwandan leader's most steadfast supporters have been Mr. Blair and Bill Clinton, who has said that one of his greatest regrets as president was not intervening to stop the genocide in 1994. 

The post-genocide regime in Rwanda has many friends around the world for understandable — and in most cases, honorable — reasons. Horrified as we were by the bloodbath in 1994, and ashamed by our inability to prevent or stop it, who would want to believe that the good face Mr. Kagame has put on Rwanda — creating an image as a prospering and healing nation — is in fact a lie? Today, opposition voices in Rwanda have been completely silenced.

Yet, it is precisely the outside world's need for a soothing moral tale — for a Manichean narrative to believe in — that betrays the reality in post-genocide Rwanda and renders us complicit, yet again, in more bloodshed. In a place where the absence of democracy and gross violations of human rights have already led to the ultimate collective crime, we simply cannot afford to continue to avert our gaze from Mr. Kagame's violent and arbitrary rule.

General Karake's case is a searing reminder of the need to hold Mr. Kagame accountable. He too belongs in the dock, standing trial before the International Criminal Court in the Hague.

Stephen W. Smith, is professor of African studies at Duke University and a former Africa editor of Le Monde and Libération.