When I teach this topic, I always recommend that students read Commandant of Auschwitz: The Autobiography of Rudolf Hoess. It is, to my mind, the most interesting autobiography ever written. If you are interested in the attitudes and inner-workings of the mind of a devoted Nazi, there are few better places to start.

Regret forms a slightly disturbing undercurrent to the whole book (mostly regret at the situation that he has found himself in), unfortunately I have left my copy at work (obviously now inaccessible) but there are repeated sections where he suggests that being forced to implement the Final Solution was a stain on the uniform of the SS. In particular he makes reference to an incident where many high-ranking SS men complained about the work they were being made to do to such an extent that Theodor Eicke (who was Inspector of Concenration Camps) had to call them all to a meeting to tell them that the role of the SS was to eliminate enemies of the Nazi State. However, it is important to distinguish here between regret at being made to do something and regret that it is being done, with most SS members almost certainly experiencing the former rather than the latter.

Hoess' main regret is that he laments that he was forced to continue working in the SS-Totenkopfverbände (the "Death's Head Units" who were responsible for running the concentration camps) because he was so good at the job and there was no one suitable to replace him. He claims that all he really wanted to do was go back to the land and work as a farmer.

Perhaps the most convoluted parts of the book are where Hoess expresses regret at the rabidness of anti-semitic feeling in Germany. He freely admits to being an anti-semite but also claims that the writings in Der Stürmer (a rabid anti-semitic tabloid newspaper) were needlessly unpleasant and pornographic and should not have been allowed. He instead suggests that anti-semitism should be allowed to triumph through the overall moral, spiritual and military victories of Nazi Germany.

Finally, Hoess claims that he regrets the suffering of the people who died in the concentration camps. He frequently suggests that had he had his wish, the horrific conditions that prisoners experienced in places like Auschwitz and Dachau would not have been allowed to continue and instead prisoners who worked would have experienced better conditions and those who were unfit for work would have been killed more humanely. He constantly claims that these conditions were the result of unhelpful superiors and incompetent subordinates. This is the part that I always stress when discussing with students - the bureaucratic evil that existed in Nazi Germany allowed men who would otherwise have been in charge of small factories or local government departments instead were given the power to end life on an unprecedented scale. Hoess regrets the means and not the ends and that is no regret at all.

Hopefully this goes some way to answering your question in a limited sense. I would like to re-state my recommendation of Hoess' autobiography but with the caveat expressed by Primo Levi in his introduction: 'this book [is] filled with evil, and this evil is narrated with a disturbing bureaucratic obtuseness; it has no literary quality and reading it is agony.'