His SS Nazi Party identification card, dated 1936, shows a sombre-looking young man with short-cropped hair and round-rimmed spectacles. His rather protruding ears add to the air of 1930s dorkiness. Just another quasi-intellectual Nazi bureaucrat, you might think. But Konrad Morgen, who a few years later rose to the rank of investigating judge in the SS judiciary, was a remarkable man who boldly took on some of the worst perpetrators of evil during the Third Reich, and attempted, in his oblique way, to curtail their crimes against Jews and other victims.

He specialised in prosecuting cases of corruption - embezzlement, double-bookkeeping, black marketeering - among the SS. In so doing, he learned of the far greater crime of the Nazi regime - the systematic extermination of the Jews.

At the behest of the head of the SS, Heinrich Himmler, his investigations took him into Buchenwald, Dachau and Auschwitz, where SS corruption was rife. There he looked with alarm into the ''abyss''.

It was beyond the scope of his powers to prosecute these mass murders because they were deemed legal under Nazi law, indeed they were ordered by his boss Himmler and ultimately by Hitler himself, who was the very embodiment of the law during the Third Reich.

But Morgen reasoned that, by going after corrupt high-ups in the SS, including concentration camp commandants, he would put them out of action and thus impede the course of the Final Solution. Naturally, this earned him powerful enemies, so he was risking his own neck.

Unlike other Nazi judges, at least he did some good

However brave, Morgen was no saint. His world-view and moral compass from within the ranks of the SS were certainly skewed. For him, the elite SS - the Schutzstaffel, or ''security squadron'' - was a noble fraternity that just had bad eggs in it. He was shocked by the rampant theft and fraud and wanted to clean it up.

What makes him such an intriguing subject of a new book - Konrad Morgen: Conscience of a Nazi Judge - is the complex make-up of his rationale, all the grey areas of his humanity. People like Morgen do the right things for the wrong reasons, they hold essentially honourable principles which are nevertheless capable of being warped by the prevailing doctrines of their times.

The son of a Frankfurt train driver, Morgen resolved to escape his humble background and practise law; he described himself as a ''fanatic for justice''. A pacifist and clearly no Nazi ideologue, he joined the SS and Nazi Party as a formality, persuaded by his mother that a career in the civil service would otherwise be closed to him.

By 1939, he was a practising judge at the district court in Stettin, then in Germany. Two years later, by now a member of the SS Judiciary - the special judicial system set up to try crimes within the SS - he was assigned to the SS court in Krakow, where he began to acquire a reputation as a dogged pursuer of venal officials and a hard-nosed "hanging judge". The first scalp he took was a relatively minor one: Georg von Sauberzweig, the chief of a troop-supply depot in Warsaw, who embezzled provisions and sold them on the black market. Morgen arrested him and he was tried, convicted and shot.

Later on, ostensibly for ''insubordination'' but mainly because his aggressive investigations into influential figures made him unpopular, his superior complained about him to Himmler, who punished him by dispatching him to the Eastern Front with the Wiking Panzer division.

But in 1943, suddenly recalled by Himmler to join the Reich Criminal Office in Berlin, his pursuit of the camp commandants began, starting with the vile Karl-Otto Koch, who ran, in turn, Sachsenhausen, Buchenwald and Majdanek. When Koch was suspected of embezzlement and profiteering, Morgen personally arrested him in the middle of the night. His audits revealed that Koch had enriched himself to the tune of some 200,000 Reichmarks, nearly £4million in today's money, largely by misappropriating the valuables of murdered Jews (including dental gold taken from corpses), which by law belonged to the Reich.

Then Morgen turned maverick. Going beyond his brief, he extended his investigation to include murder. He discovered that Koch had developed a system of eliminating all the inmates who had witnessed his corruption, disguising these killings with fictitious medical records stating that they had died of natural causes. He also killed off two hospital workers who had treated him for syphilis, so as to keep his condition secret. Astonishingly, within an organisation which helped to run the machinery of the Holocaust, Morgen managed to prosecute a senior SS figure for the illegal murder of Jews. Tried and sentenced to death, Koch was executed by SS firing squad on 5 April 1945.

In his reports to the SS Judiciary, Morgen showed genuine sympathy for Koch's victims. Unlike the other Nazi judges, he never made derogatory remarks about Jews, and he denounced their mistreatment. But while he was sensitive to deliberate acts of cruelty and abhorred criminal behaviour, he was unmoved by more generalised Jewish suffering, sanctioned by the state. He didn't question the disenfranchisement and subjugation of the Jewish populations of Europe, or their wholesale deportation to the camps. He just wanted the camps to be run with rectitude. Morgen's concern was for criminal justice, not social justice.

Next, Morgen turned his attention to Auschwitz, looking into the doings of the sadistic head of the Auschwitz Gestapo, Maximilian Grabner, who'd had 2,000 prisoners shot solely to make room in the arrest bunker when it became overcrowded. These killings, too, were covered up by fake medical certificates. Morgen shrewdly persuaded Gestapo Chief Heinrich Muller in Berlin that these "unauthorised" killings were a challenge to his, Muller's, authority, i.e. illegal, and that Grabner should be prosecuted.

Years later, as a witness at the Nuremberg Trials, he explained his tactic: ''I saw a way open to me by way of justice - by removing from this system of destruction the leaders and important elements through the means offered by the system itself. I could not do this with regard to the killings ordered by the state, but I could do it for killings outside of this order… I started proceedings against these men, and this would have led to a shake-up of the system and its final collapse.''

As we know, Morgen's crusade didn't cause the concentration camp system to collapse. Still, his limited success must have given this ''fanatic for justice'' some satisfaction. Although Grabner's 1944 prosecution failed (it wasn't until 1948 that he was finally convicted and hanged in Poland), he got a better result with Hermann Florstedt, the Majdanek commandant who embezzled gold, money and furs taken from Holocaust victims, and killed the witnesses. The SS executed him in April 1945.

After the war, Morgen spent three years in American custody, being interrogated by the Counter-Intelligence Corps (CIC), before establishing himself as a lawyer in Frankfurt. Throughout subsequent decades, he testified at numerous war-crimes trials, and while accepting the enormity of Nazi atrocities, continued to defend the ideal of the SS, as he saw it, with its universal virtues of honesty and integrity.

Blind to the organisation's inherently vicious role, he condemned only those individuals who had ''betrayed'' it. He died in 1982.

Perhaps, in the end, what he tried to achieve under the Nazi regime was akin to ''straightening the pictures in a madhouse'', as the authors of the book about him put it. But at least he did some good, which is a lot more than can be said for his fellow Nazi judges.

'Konrad Morgen: Conscience of a Nazi Judge', by Herlinde Pauer-Studer and J. David Velleman, is published by Palgrave Macmillan at £20.