Ordinary Men might be the one you’re thinking of? It’s about Reserve Police Battalion 101 in Poland, and largely about how ordinary people come to be monsters in certain situations.
So… I probably won’t be able to find many accounts, then?
For fairly obvious reasons, there have been relatively few voluntary accounts provided by such perpetrators. (There are plenty of statements about their careers that were provided by probable perpetrators to legal authorities over the years, but those statements frequently seek to obscure the truth about their involvement and so are presumably not what you have in mind.)
Of those few, some do appear to be what you’re looking for: former camp personnel trying to honestly explain the mindset and their involvement. The best living example of this is probably Oskar Gröning, recently mentioned in this thread. But he’s a rare case. Indeed, with reference to that thread, if there were any other Auschwitz guards still alive and willing to speak, we could have expected Rees to have unearthed them for his BBC series, but Gröning was about it. Rees’s previous series have featured interviews with perpetrators from Nazi atrocities elsewhere; as rule, those interviewees have been invariably unrepentent.
There have been those who have provided accounts, but where there have been suspicions that these have been self-serving. Thus a young SS guard at Auschwitz, Pery Broad, wrote an account of his time at the camp while in British captivity after the war. It’s regarded by historians as valuable, but problematic in that it’s probably coloured by Broad attempting to ingratiate himself with his captors. That’s available as part of KL Auschwitz Seen By The SS (Interpress, 1981), a compilation of the three significant contemporary (or nearly so) autobiographical documents about the camp written by those running it. The best known of these is the memoir by the commandant Rudolf Höss, which has been frequently reprinted in multiple editions, while the other is the diary kept by Johann Kremer, one of the doctors. Neither may be quite what you had in mind.
Gitta Sereny is always worth reading and that’s a fine collection of some of her journalism, but in the specific cases she covers in it there are more thorough accounts of the individuals concerned elsewhere. Thus the chapter based on interviewing Franz Stangl, the commandant at Treblinka, in prison is actually only the original 1971 article that she subsequently expanded into a full-length book in 1974 about him, her classic Into That Darkness. There’s also the brief piece she wrote about Dr Hans Münch, the SS pathologist who served in Auschwitz but who refused to take part in “selections”. In that instance, he was interviewed at much greater length by Robert Jay Lifton for The Nazi Doctors (Basic, 1986), which puts his career in deeper context - he’s the individual masked by Lifton as “Ernst B.”
Even if the surviving prepetrators have generally been unwilling to talk and are now all old and frail, some are still out there. The highest profile is probably Aribert Heim, the former Mauthausen doctor currently on the run after having been tracked to Spain last year. At a somewhat more mundane level, Scotland Yard has just reopened its investigation unit looking at possible Nazi war criminals who settled in the UK - the numbers involved are now a few dozen and mainly Ukrainians accused of atrocities in the East.