I've got 2 suggestions for you. The first thing you might want to check out is a book called Ordinary Men, by Christopher Browning (Amazon link). It's a book about German soldiers who committed Nazi atrocities during World War 2, and like the title suggests, it talks about how these perfectly "ordinary men" came to start seeing the things they were doing as normal and necessary. There are some interesting parts where these men try to explain and understand how they came to see brutality as acceptable.

The other thing you might be interested in is a documentary called "The Act of Killing" (It's now here on Netflix streaming). It's a nominee this year for an academy award for Best Documentary. I haven't gotten around to seeing it yet, but in it a documentary filmmaker finds some men who led death squads in Indonesia and asks them to make their own films re-enacting what happened from their perspectives. These are men who killed and tortured innocent people in horrible ways and are still living freely because the government supported what they did at the time. I've heard nothing but fabulous things about it and I think it might be right up your alley. Quick note: it's a new documentary, but the Indonesian killings occurred in the mid 1960s, so I think this falls inside the timeline that Ask Historians aims to focus on.

Vasili Blokhin , the person who executed the most number of people single handedly in history (which included 7000 Polish prisoners alone during the Katyn massacre) reportedly went mad after his enforced retirement upon the death of Stalin, ending up an alcoholic and commiting suicide in 1955.

Stalin And His Hangmen by Donald Rayfield also has several anecdotes about various Cheka toturers who went round the bend somewhat during the 1920s such as a guy called Saenko working in Kiev, who suffered a breakdown and ended up attacking his superiors, and Dr Mikhial Kedrov, who was a close friend of Lenins and whose family had a history of mental illness, being singled out by Rafield as especialy cruel.

The Drowned and Saved by Primo Levi, more specifically The Grey Zone (a chapter within the book) details how men in the concentration camps who were part of the SonderKommandos (the men in the "cleaning squads") went about their duties and how it affected them.

Basically the men were not actual Nazis but rather people within the camp itself chosen by the SS to do the duties of removing the dead from the gas chambers, cutting their hair, removing gold teeth, burning/burying the bodies etc.

The men were given the choice of either dying or working and in working they no longer were innocents but became accomplices in the horrors of the camp. They justified their actions with phrases along the lines of "if we didnt do it, someone else would" while drinking heavily to forget.

Primo describes how over time the men no longer resembled men but rather descended into their basic instincts in order to survive where what we consider barbaric and horrific became the norm.

Meister Franz, performed executions and other punishments from 1573 to 1617 in a Bavarian province. He kept a diary recording 361 executions and 345 minor punishments.

American Academy in Berlin: "Although the original volume is no longer extant, several manuscript versions of it circulated during the subsequent two centuries. Three published versions appeared during the hundred years after that, the last in 1928."

Excerpt:

July 28 th [1590]. Friedrich Stigler from Nuremberg, a coppersmith and executioner’s assistant. For having brought accusations against some citizens’ wives that they were witches [ literally Druids ] and he knew it by their signs. However, he wittingly did them wrong. Also said that they gave magic spells to people. Likewise for having threatened his brother, the hanged Peterlein, on account of which threat he had appeared before the court at Bamberg several years ago, but was begged off. Lastly, for having taken a second wife during the life of his first wife, and a third wife during the life of the second, after the death of the first. Executed with the sword here out of mercy. November 6 th [1595]. Hans Sigert, from Pollingen near Neuenmark, a farm-hand who murdered a tailor named Summerstein at the Sünderspühl with a fence post (zaunstecken). Executed here with the sword. Wept all the way until he knelt down. November 13 th [1617]. Burnt alive here a miller of Manberna, who however was lately engaged as a carrier of wine. Because he and his brother, with the help of others, practiced coining and counterfeiting money and clipping coins fraudulently. He also had a working knowledge of magic…This miller, who worked in the town mills here three years ago, fell into the town moat on Whitsunday. It would have been better for him if he had been drowned, but it turned out according to the proverb that “What belongs to the gallows cannot drown in water.” This was the last person whom I, Master Frantz, executed."

Joel Harrington found an extant first edition for his book The Faithful Executioner. Reviewing the book, Hilary Mantel stated "Exploring both sixteenth-century Nuremberg and the world about the city, [Harrington] re-creates the social context for the flamboyant displays of cruelty that later centuries find so hard to comprehend. Both the executioner and his victims are rescued from our condescension and restored to their own moral universe."