Were concentration camp Kapos sometimes Jewish prisoners, or were they always non-Jewish convicts and political prisoners?

Some of the concentration camp Kapos were indeed Jewish inmates; others were not.  Regardless of their ethnicity, all of the Kapos were under pressure to perform whatever function was assigned to them due to the numerous possible punishments--one of which included simply being nailed to a box and left for dead.  This, however, did not stop the prosecution of Kapos as war criminals following the fall of the Third Reich.  During the trials, the Kapos were treated as if they were complicit in the actions they had carried out in the camps--a dilemma that still haunts Holocaust studies.

The point of the Kapos was, of course, to instill constant fear in the inmates, even of each other.  This also took place outside the camps with the use of Block Wardens, those who were lowest on the Nazi hierarchy and responsible for reporting any "suspicious" activity.  But anyone could have been an informant. In fact, people sometimes reported others simply to be rid of annoying neighbors or competing small business owners.