In May 1960, Adolf Eichmann was kidnapped in Argentina by Mossad agents and spirited to Israel. The former SS Obersturmbannführer, who had been Heinrich Himmler ’s chief adviser on “Jewish affairs,” was subsequently tried in Jerusalem at proceedings that captured the interest of the world—especially that of a German-American political scientist named Hannah Arendt, herself a Jewish refugee.

For Arendt, the Eichmann case raised existential questions about the nature of evil. As she wrote in five articles for the New Yorker (which became the 1963 book called “Eichmann in Jerusalem”), she had expected to encounter a monster. Instead, she saw an unimpressive, elderly man on public display in a bulletproof glass box. More unsettling to Arendt were Eichmann’s claims that he was just following orders and that responsibility for the Holocaust lay with his superiors. He portrayed himself as a small cog in the large wheel of systematic mass murder, or as he put it to the court: “a tool in the hands of stronger powers.” The court disagreed, finding Eichmann guilty of crimes against humanity. He was hanged on May 31, 1962.

Arendt became fixated on the disturbing gap between the seeming nobody she watched in the clear box and that nobody’s central role in the murder of millions. She captured this chasm in the famous phrase “the banality of evil,” by which she meant to imply that Eichmann was not a fanatical anti-Semite but an unthinking bureaucrat. She depicted him as a rather minor figure who never really thought about what he was doing. “The trouble with Eichmann,” she wrote, “was precisely that so many were like him, and that the many were neither perverted nor sadistic, that they were, and still are, terribly and terrifyingly normal.”

In “Eichmann Before Jerusalem: The Unexamined Life of a Mass Murderer,” German philosopher Bettina Stangneth joins the growing number of scholars who have challenged Arendt’s claim. Her argument is that Eichmann was indeed evil, but far from ordinary. She makes this case by focusing not on the old man who appeared in court but on what Eichmann actually said and did as a free man in between the end of the war and his capture in Argentina.

At the core of Ms. Stangneth’s book is a careful analysis of conversations Eichmann had—and notes he made—while he was living under an assumed identity in Buenos Aires between 1950 and 1960. In 1957, Eichmann joined a meeting group of former SS members hosted by Willem Sassen, a Dutch Nazi collaborator and journalist. Their conversations were taped and transcribed so that Sassen could use them in a book he was planning to write denying Nazi war crimes. But Eichmann’s words do just the opposite. His statements reveal a man who took part in the genocide knowingly and actively.

On the Sassen recordings, Eichmann never expresses regret for anything. On the contrary: “I have to tell you quite honestly that if of the 10.3 million Jews . . . identified, as we now know, we had killed 10.3 million, I would be satisfied, and would say, good, we have destroyed an enemy,” he says. Some of the transcripts were published in American and German newspapers and made their way to Eichmann’s prosecutors in Jerusalem, but the court allowed only a small portion of them to be presented at trial. (The actual recordings were not available to the court and their authenticity, therefore, couldn’t be verified.) Ms. Stangneth, acting more like an investigative journalist than an academic philosopher, does an excellent job in tracing the odyssey of these archival records, which are scattered across various continents.

During the trial, Eichmann denied that the conversations were genuine. When his handwritten notes on the typed transcripts made such denial difficult, he dismissed his statements as “tavern talk.” Arendt accepted this image of a drunk nostalgic Nazi boasting about the past. But Ms. Stangneth, drawing on documents and research that Arendt never had access to, reaches a different conclusion: He was a master manipulator. “Whether he was in the Third Reich, Argentina, or Israel, Eichmann gave detailed and well-informed accounts of the murder of millions. He simply adjusted the account of his own role, and his attitude toward the murders, to his changing circumstances,” she writes.

Ms. Stangneth, drawing on research by the Argentinian author Uki Goñi and others, also reminds us how openly the networks of former Nazis operated and how far they reached. Like thousands of other Nazis and collaborators, Eichmann escaped to Argentina with the help of Italian Catholic priests and Argentinian officials while carrying Red Cross travel papers. Ms. Stangneth emphasizes the lack of interest Allied authorities showed in bringing former Nazis to justice after the war. This is consistent with my own research, which shows that the Nazis’ escape networks were well-known by many governments and institutions, including the U.S. State Department, as early as 1947. But with the increasing tensions between the West and the Soviet Union, denazification efforts became less and less important. After the Korean War broke out in 1950, attention almost completely shifted to the new enemy: communism. It now appears that the German intelligence service was aware of Eichmann’s whereabouts as early as 1952 but showed little effort to apprehend him. Only Israel was willing to take justice into its own hands.

With her well-written and impressively well-researched book, Ms. Stangneth not only adds many new, surprising details to our picture of Eichmann before the trial but also prepares the stage for follow-on research. Numerous Eichmann files compiled by the German Intelligence Service remain classified, and legal battles to fully declassify them are still being fought in German courts. When we finally gain access to these and other documents, we will come even closer to the truth about Eichmann and the rest of Hitler ’s henchmen.