This trope of the sadomasochistic, leather whip wielding SS-woman/commandant's wife is a post-war trope, not one invented during the war.

I have written about gender and the Holocaust and go a bit into this but to expand on this: The cruel woman trope in connection with the camps does not have an empirical basis beyond the fact that female guards of the SS-Auxiliary were as far as we now know as cruel and brutal as their male counterparts, which is to say rather very much.

But there is a distinct treatment of female guards and despite their relatively low number, they have become a focal point of imagination in certain media, from the Stalagim novels to Italian exploitation movies a la Ilse, She-Wolf of the SS and others. Now, there were women who are remembered for their cruelty such as Irma Grese and more than probably any one else, Ilse Koch, wife of Karl-Otto Koch, the commandant of the Buchenwald concentration camp and dubbed "the witch of Buchenwald". Koch was accused with a variety of brutal behavior, some of which might have been fantasy, and was in the end sentenced for cruelty towards the prisoners.

The important thing about the Grese trial and the trial of Ilse Koch was that they captured the public's imagination with regards to the SS-women/commandants' wifes. Their portrayal as beasts and monsters, even more so than in the case of many of their male colleagues, had a lot to do with the gender discourse framing of femininity at the time. Virtually all Western societies, not the least the German, viewed women ideally as passive care-givers and gentle. When it came to light that women were as much able to be as cruel and brutal as their male colleagues when it came to the camps, the narrative that was created to reconcile the pervasive framing of women as passive with the apparent brutality they exercised was to make them out to be deviant in some way. The excessive and often sadomasochistical sexuality that was made out in these women as well as them being either painted as very ugly or seductively beautiful are part of this narrative that seeks to set them apart from "normal" women.

Even further, the idea that doing evil must somehow manifest in a physical defect (Hitler's balls, anyone?) or in being sexually deviant is older than the Nazis and almost dates back to pre-modern times but is heavily employed here. It still is in fact, if anyone has read Jonathan Littell, who makes the Nazi in his book have incestuous desires in order to "explain" his Nazi-ness. The popularity of this idea that was also influenced by a pop-reading of Freudian psychology, serves not so much the purpose of making it easier to fight people but rather as an easy explanation for the cruelty and brutality displayed by these people. By banishing the Nazis to the realm of the sexuality deviant, the psychotic, the social aberrant, we stifle the need for further historical and psychological explanations for their behavior. By making them crazy, by placing them outside the realm of the understandable, we are not forced to engage with them on a deeper level.

Another factor that influenced this portrayal is a certain reading of fascism and sexuality. Susan Sonntag has discussed this topic several times, arguing about death and lust, about sexuality and taboo, and about the fascination of fascism. This found an outlet in movies such as the Night Porter, which portrays the sadomasochistical relationship between a female survivor and her former tormentor under inverted circumstances or in Pasolini's Salo or the 120 days of Sodom, where taking de Sade's 120 days of Sodom as a source material, fascism is portrayed as the escalation of corruption, sexual depravity, and violence.

In conclusion, there were cases of female SS-guards and commandants' wifes behaving in a cruel and brutal fashion but the sexual politics of these post-war portrayals are almost wholly imposed by the gender politics and discourse of the post-war era and perception.