I'm interested in knowing how the Nazi regime went about hiring the guards and other camp personnel who did the dirty work.
Were they screened for sadism, callousness, obedience, or some other trait that was conducive to the job, or did they not do much about it until someone turned out to be too gentle, compassionate, or principled to dish out the abuse?
Related, was it ever a practice to parole criminals in exchange for their obedience (sort of a, "Do as you're told and we'll let you abuse all of the victims you want")?
Short answer - it was less "hiring" and more "drafting". No pre-screening happened that I could find evidence of (well, except for the general "has to be a good Nazi" principle, which might count towards that), but being soft on incarcerated was strongly discouraged, which led to a predictable result of "refining" the most harsh guard possible.
The camps were primarily guarded by the SS-Totenkopfverbände - a special division of the SS. One didn't join the camp guards - one joined the SS. At that point, it didn't really matter what one wanted - members of the SS were expected to follow orders as much as soldiers were. If you were assigned to SS-Totenkopfverbände, you went to guard the camp.
While technically there were no "screening for sadism", SS were comprised of people who either truly accepted Nazi ideology or successfully faked it. Thus, they indeed were "pre-screened" for being 1) obedient (because of patriotism), 2) dismissive towards certain groups of people. As concentration camps held people who a Nazi would consider subhuman, the treatment of inmates would be… not good. In fact, Theodor Eicke, commandant of Dachau (the first concentration camp and the model for subsequent camps), encouraged his people to treat inmates with "inflexible harshness": they were the enemies of the state, after all! And that attitude was replicated in other camps - the Dachau camp was a training facility for the SS guards. As such, anyone who completed that training would appropriate the behavior of his SS teachers.
In the last days of WW2, SS formed so-called "SS-Mannschaft" (Auxiliary SS) - a ragtag bunch of personnel drafted from Volkssturm, Army and basically any other source SS could get people from to try and keep camps running to the last moment while personnel of the SS proper could escape. These troops did not go through the usual SS selection process and did not get the same training as SS-TV personnel, so their behaviour could be very varied. But due to the chaos caused by quickly progressing Allied offensives, almost no documentation exist on what happened in camps in those days.
P.S. While searching for sources for this answer, I kept finding stories about dismissal of guards deemed "too kind" to prisoners. I couldn't track down any of these to a reliable source, but there might be some truth to them. If true, it would serve to further reinforce the Eicke principles.
P.P.S. Regarding your "parole to criminals" question - they weren't drafted to be camp guards, but you might want to read up on Strafbatallions (Army version, pretty tame and generally drafted from minor offenders) and Dirlewanger Brigade (SS version. It was initially supposed to be drafted from poachers. That idea was quickly lost, and the unit was drafted from murderers, burglars, criminally insane and so on. It had its debut during the occupation of Poland, and quickly earned itself the reputation of being a congregation of slaughterers, looters and rapists. Believe or not, but things went downhill from there.).
Комендант Освенцима. Автобиографические записки Рудольфа Гесса. (unofficial translation of "Commandant of Auschwitz : The Autobiography of Rudolf Hoess", Deutsche Verlags-Anstalt, Gmbh., Stuttgart, 1958)
Koehl, Robert, "The SS: A History 1919-45", Stroud: Tempus, 2000
This question is almost a mirror of the more usual one about the defense that supposedly "decent" people were committing atrocities only under duress. The answer to that question is that
- usually people could avoid participation in atrocities,
- doing so might limit their promotion chances, or put them to the frontlines where armed enemies would shoot at them,
- they may or may not have been aware that it was relatively easy to avoid participation.
Regarding your question, after the chaotic early years the Nazis were trying to organize the genocide in a way that would slow the mental deterioration of their troops, which was a concern and prompted the switch from mass shootings to the gas chambers. Read Himmler's speech in Posen and perhaps Goldhagen's book about the police battalion 101 (also read the criticism of Goldhagen's views). The Nazis were looking for people to operate an "industrial" killing operation, not undisciplined people who slow the lines with personal atrocities.
Actually, the horrific tasks of herding gassing victims into shower rooms, gassing them, and moving bodies into cremation ovens, was performed by "Kapos" in the camps, who were themselves inmates, Jewish inmates too. The SS organized prisoners into a hierarchical structure of tyranny in which they played inmates against each other, and had inmates perform the detailed tasks of the gassings. Kapos who were reliable evidently lived longer, but they too could be murdered at any time… delaying inmates' executions was an extortion tactic that the SS used to recruit those Kapos.List of site sources >>>