Dr. Mengele was nicknamed the "Angel of Death" by the prisoners because he had the face of an angel, yet he callously made selections for the gas chambers at Birkenau. He was nice to the children in the camp, yet he experimented on them as though they were laboratory rats. He volunteered to do the selections at Birkenau, even when it wasn't his turn, because he wanted to find subjects for his medical research on genetic conditions and hereditary diseases, which he had already begun before the war. He particularly wanted to find twins for the research that he had started before he was posted to Birkenau.
The first killing center set up in occupied Polish lands was the camp at Chełmno on the Ner; Jews brought in from the ghettos in the Wartheland were being killed there from December 1941. Three more camps, somewhat larger, were opened at Bełżec, Sobibor, and Treblinka (in what was known as “Aktion Reinhard”) somewhat later, between March and July 1942.

In February 2010, a 180-page volume of Mengele's diary was sold by Alexander Autographs at auction for an undisclosed sum to the grandson of a Holocaust survivor. The unidentified previous owner, who acquired the journals in Brazil, was reported to be close to the Mengele family. A Holocaust survivors' organization described the sale as "a cynical act of exploitation aimed at profiting from the writings of one of the most heinous Nazi criminals".[118] Rabbi Marvin Hier of the Simon Wiesenthal Center was glad to see the diary fall into Jewish hands. "At a time when Ahmadinejad's Iran regularly denies the Holocaust and anti-Semitism and hatred of Jews is back in vogue, this acquisition is especially significant", he said.[119] In 2011, a further 31 volumes of Mengele's diaries were sold—again amidst protests—by the same auction house to an undisclosed collector of World War II memorabilia for $245,000 USD.[120]

Twins were subjected to weekly examinations and measurements of their physical attributes by Mengele or one of his assistants.[49] The experiments he performed on twins included unnecessary amputation of limbs, intentionally infecting one twin with typhus or some other disease, and transfusing the blood of one twin into the other. Many of the victims died while undergoing these procedures,[50] and those who survived the experiments were sometimes killed and their bodies dissected once Mengele had no further use for them.[51] Nyiszli recalled one occasion on which Mengele personally killed fourteen twins in one night by injecting their hearts with chloroform.[31] If one twin died from disease, he would kill the other twin to allow comparative post-mortem reports to be produced for research purposes.[52]

Auschwitz, perhaps the most notorious and lethal of the concentration camps, was actually three camps in one: a prison camp (Auschwitz I), an extermination camp (Auschwitz II–Birkenau), and a slave labour camp (Auschwitz III–Buna-Monowitz). Upon arrival, Jewish prisoners faced what was called a Selektion. A German doctor presided over the selection of pregnant women, young children, the elderly, handicapped, sick, and infirm for immediate death in the gas chambers. As necessary, the Germans selected able-bodied prisoners for forced labour in the factories adjacent to Auschwitz, where one German company, IG Farben, invested 700 million Reichsmarks in 1942 alone to take advantage of forced labour, a capital investment. The conglomerate presumed that slave labour would be a permanent part of the German economy. Deprived of adequate food, shelter, clothing, and medical care, these prisoners were literally worked to death. Periodically, they would face another Selektion. The Nazis would transfer those unable to work to the gas chambers of Birkenau.
Anxious to limit immigration to the United States and to maintain good relations with the Vichy government, the State Department actively discouraged diplomats from helping refugees. However, Bingham cooperated in issuing visas and helping refugees escape France. Hiram Bingham gave about 2,000 visas, most of them to well-known personalities, speaking English, including Max Ernst, André Breton, Hannah Arendt, Marc Chagall, Lion Feuchtwanger and Nobel prize winner Otto Meyerhof.
And here is where the enduring relevance of the Harbonah story comes in. Just a few weeks ago, for instance, the vexed question of Polish collaboration in the Holocaust was once again in the headlines, the subject of a diplomatic fracas between Jerusalem and Warsaw. Surely the recent efforts by the Polish government to distort or cover up the historical record are deserving of sharp criticism, and the hundreds if not thousands of Poles who aided in the extermination of the Jews deserve ignominy no less than did the thousands of ancient Persian subjects who volunteered to help Haman.

Medical experiments conducted on camp inmates by the SS were another distinctive feature.[51] At least 7,000 prisoners were subjected to experiments; most died as a result, during the experiments or later.[52] Twenty-three senior physicians and other medical personnel were charged at Nuremberg, after the war, with crimes against humanity. They included the head of the German Red Cross, tenured professors, clinic directors, and biomedical researchers.[53] Experiments took place at Auschwitz, Buchenwald, Dachau, Natzweiler-Struthof, Neuengamme, Ravensbrück, Sachsenhausen, and elsewhere. Some dealt with sterilization of men and women, the treatment of war wounds, ways to counteract chemical weapons, research into new vaccines and drugs, and the survival of harsh conditions.[52]

Concentration camps began to incarcerate ‘habitual criminals’ in addition to political prisoners. Goebbels stepped up anti-Semitic propaganda with a traveling exhibition which cast Jews as the enemy. Nearly half a million people attended. Some guessed worse would come. Winston Churchill criticised British relations with Germany, warning of ‘great evils of racial and religious intolerance’, though many colleagues complained of his ‘harping on’ about Jews.

According to psychologist Eva Fogelman “Rescuers’ families nourished an independence of mind and spirit. … In talking with rescuers from all kinds of different homes, I found that one quality above all others was emphasized time and again: a familial acceptance of people who were different. This value was the centrepiece of the childhood of rescuers and became the core from which their rescuer self evolved. From the earliest ages, rescuers were taught by their parents that people are inextricably linked to one another. No one person or group was better than any other. The conviction that all people, no matter how marginal, are of equal value was conveyed to children of both religious and nonreligious households.” Fogelman is the daughter of a Holocaust survivor and founder of the Jewish Foundation for Christian Rescuers.

In 1942, Auschwitz II (Birkenau), originally intended to house slave laborers, began to be used instead as a combined labor camp and extermination camp.[23][24] Prisoners were transported there by rail from all over German-occupied Europe, arriving in daily convoys.[25] By July 1942, SS doctors were conducting "selections" where incoming Jews were segregated, and those considered able to work were admitted into the camp while those deemed unfit for labor were immediately killed in the gas chambers.[26] The arrivals that were selected to die, about three-quarters of the total,[a] included almost all children, women with small children, pregnant women, all the elderly, and all of those who appeared (in a brief and superficial inspection by an SS doctor) to be not completely fit and healthy.[28][29]