First, we should realize that the “brutal treatment” of many of the people who came to Auschwitz did not last very long. One part of the Auschwitz complex was used simply as a killing center. Jews (and some others) brought here were never really prisoners at Auschwitz. Instead, they were simply sorted out and all those who were not deemed able to work were killed in the gas chambers right away. It is true that they were often beaten and abused as they were herded into the gas chambers. However, they were killed soon after arrival and their experience was different from that of the prisoners who were selected to work.
Part of the point of Auschwitz was to get some productive labor out of the prisoners before they died. There tended to be some degree of conflict between the SS and the people who were trying to produce goods (notably synthetic rubber and fuels) using the Auschwitz prisoners. The SS were more interested in killing the Jews and some others while the producers wanted to get more labor out of them. The SS generally won this conflict, which meant that the prisoners were worked very hard while being given little food. They were kept in terrible conditions. Their experience, then, was of slow starvation and deterioration while being forced to work hard. Their experience often ended with them being killed in the gas chambers when they became too weak to work.
Thus, the brutal treatment of Jews and other prisoners generally came in two types. They could be brutally killed right away or they could be worked hard and treated very badly until the either died or were killed. Either way, it was a horrible episode in human history.