How did Kirschbaum die?

There are a few different answers to this question. First, when questioned, Preuss tells the narrator that Dr. Kirschbaum took his own life, using poison he carried in his medical bag. The second version of Kirschbaum's death is that after Sturmbannführer Hardtloff died as a result of a heart attack (Kirschbaum was taken to care for him), he was then killed because he was unable to save the man's life. In the end, we cannot be sure whether Kirschbaum's death was suicide or murder.