The Nazi officer in charge of the Ormaie Gestapo, Julie's interrogation, and her confession, von Linden is a man in a tight spot, and despite the fact that he's clearly made some bad life choices, we can't help but feel a little sorry for him in the end. He's representative of the type of person who probably goes along with the Nazi regime for the sake of the order it provides—Julie describes him by comparing him to a character from Peter Pan:
Von Linden resembles Captain Hook in that he is rather an upright sort of gentleman in spite of his being a brute, and I am quite Pan-like in my naïve confidence that he will play by the rules and keep his word. (1.8.XI.43.7)
Later, Julie finds out that in civilian life he was the headmaster of a boys' school, and indeed, his interest in literature and opera indicate that he's a well-educated, cultured guy, like so many of those Nazis were.
While he doesn't turn all the way like Engel, he does appear to have a real crisis of conscience as time goes on—the letter from his superior officer indicates that he has been protecting Julie, or at least not actively seeking to punish her. For example, there's evidence that he knows she's not telling him one thing of military value in her "confession." His ultimate suicide indicates that he could no longer live with something, and since he's the head of the Ormaie Gestapo, we can pretty much take our pick of what that something is.