SS-Obersturmführer Johann Paul Kremer, M.D., Ph.D., professor at the University of Münster, arrived at Auschwitz on August 30, 1942, where he replaced a doctor who had fallen sick. He carried out assessments of prisoners attempting to gain admission to the hospital. 

Kremer ordered most of them killed by phenol injection. He selected prisoners who struck him as particularly good experimental material, and questioned them just before their deaths, as they lay on the autopsy table awaiting injection, about such personal details as their weight before arrest and any medicines they had used recently. In some cases, he ordered these prisoners photographed.

He witnessed gassings in Auschwitz and wrote about them in his diary:

September 2, 1942
For the first time, at 3:00 A.M. outside, attended a special action. Dante's Inferno seems to me almost a comedy compared to this. They don't call Auschwitz the camp of annihilation for nothing!

September 5, 1942
In the morning attended a special action from the women's concentration camp (Muslims); the most dreadful of horrors. Master-Sergeant Thilo (troop doctor) was right when he said to me that this is the anus mundi. In the evening towards 8:00 attended another special action from Holland. Because of the special rations they get a fifth of a liter of schnapps, 5 cigarettes, 100 g salami and bread, the men all clamor to take part in such actions. Today and tomorrow (Sunday) work.

After the war, Johann Paul Kremer testified about his diary. An extract is found in "The Good Old Days": The Holocaust as Seen by Its Perpetrators and Bystanders, Ernst Klee, Willi Dressen, and Volker Riess, Eds., 1991, p. 258:

Particularly unpleasant was the gassing of the emaciated women from the women's camp, who were generally known as 'Muslims'. I remember I once took part in the gassing of one of these groups of women. I cannot say how big the group was.

When I got close to the bunker [I saw] them sitting on the ground. They were still clothed. As they were wearing worn-out camp clothing they were not left in the undressing hut but made to undress in the open air.

I concluded from the behavior of these women that they had no doubt what fate awaited them, as they begged and pleaded to the SS men to spare them their lives. However, they were herded into the gas chambers and gassed.

As an anatomist I have seen a lot of terrible things: I had had a lot of experience with dead bodies, and yet what I saw that day was like nothing I had ever seen before. Still completely shocked by what I had seen I wrote in my diary on 5 September 1942: 'The most dreadful of horrors. Hauptscharführer Thilo was right when he said to me today that this is the anus mundi', the anal orifice of the world.

I used this image because I could not imagine anything more disgusting and horrific.

SS-Doctor Kremer at a hearing on 18 July 1947 in Cracow.