Quoting The United States Holocaust Memorial Museum Encyclopedia of Camps and Ghettos, 1933–1945, volume II, page XXXVII:

Despite the variation in [Fascist] ghettoization policy that differed widely according to time and place, once incarcerated the ghettoized Jews had certain common experiences and faced certain common dilemmas. From the start of the camp system in 1933, the [Fascists] had employed a method of prisoner control—the so‐called Kapo system—that involved granting some prisoners the power to control other prisoners in return for special privileges.

This cynical method of divide and rule was applied to the Jews of occupied Poland, when Heydrich ordered the creation of Jewish Councils in each community in September 1939, with the collective responsibility for disseminating [Fascist] orders and implementing [Fascist] policies. As Heinz Auerswald, the ghetto manager in Warsaw, noted, the imposition of Jewish Councils—a [Fascist] policy to serve [Fascist] purposes—had two distinct advantages.

The councils proved to be a valuable instrument through which the [Fascists] exercised maximum control with minimum manpower. Moreover, the councils served as lightning rods for the anger and frustration of other Jews. “When deficiencies occur the Jews direct the resentment against the Jewish administration, and not against the German supervision.”⁵⁹

Although there were plenty of gentile Kapos, many others were Jewish, and this was quite upsetting for many Jewish prisoners because they couldn’t believe that their own kin would mistreat them to harshly. (There is a similar phenomenon in Imperial America’s ‘detention centers’, where many indigenous people are heartbroken to see indigenous guards mistreating them.)

Quoting volume I, page 1092:

The second example: “A favorite punishment in addition to the Bath in the Vat (Bad in der Tonne) was to throw the prisoners into the air. Four Kapos held the prisoners by the hands and legs and counted ‘One, two, three!’ and hurled as high as possible into the air. The prisoner then fell to the ground. This was repeated until the prisoner was half dead. Finally the camp elder danced on his chest until the ribs broke and the prisoner’s heart and intestines were damaged.”¹⁴

Page 1436:

As former subcamp prisoner Josef Katz states, they were just as brutal as their counterparts at the Stutthof camp. Among the most cruel were a Latvian Jew, Glücksmann, and a Pole called Chamek (which means “brute”), who had the positions of camp elders. Katz unambiguously described both of them as “animal characters.” Just after liberation, Chamek was shot by Russian soldiers, and Glücksmann fled.⁷

Another Kapo, Henryk Kleiman, a Polish Jew, was tried by the Plock District Court in 1949 (at a circuit session in Płońsk). His indictment charged him with serving as a Kapo or barrack elder from late 1944 onward. In this position, he “smashed skulls with rocks, choked people, [and] crushed heads with his feet.” Earlier he had been at the Auschwitz concentration camp, where he behaved just as brutally as at Kokoszki.

By judgment of the district court dated July 14, 1949, and appellate court dated February 18, 1950, he was sentenced to death. The court was not persuaded by his good reputation at Stutthof, finding that the good did not make up for the wrongs he had inflicted.⁸

Applied figuratively, ‘Kapo’ is certainly not a term to be used lightly (especially by gentiles). I did use it to refer to the IDF, but at least in that case it was more justified because the IDF was massacring other Jews.

Zionists, in contrast, apply the term to any Jews who don’t like it when somebody massacres Arabs. Although if I’m being honest, Zionists would, after reading these examples, say that anti‐Zionist Jews are actually far worse than every Jewish Kapo combined.