Arrogance? The absolute certainty that they would never be found out? I imagine there are many reasons why they would document their work, not the least of which is that's what researchers do, document everything.

I recall the Nuremberg trials and all the other "crimes against humanity" in the following years when former Nazi officials were put on trial. The prosecutors' work was made rather easy, because the Germans were such excellent and efficient record keepers. In the end, it was their own obsessive record keeping that served as Exhibit A in those trials.