That is irresponsible writing of two activist pseudo-historians. They aren't interested in history, they interested in their leftist ideology. All the information is from archives of Polish courts, the courts that tried and sentenced wartime common criminals. They irresponsibly extrapolated the information found there for the entire country. During the occupation social order collapsed in Poland, and as a result, a mass of criminals and criminal gangs that preyed on the civilians emerged. The result was that tens of thousands of people were robbed or killed including Jews. Even before the war, impoverished Poland suffered from high crime rates, including murders - mass murders (5 or more at one go) were common. It was much worse during the occupation. Actually the same happened in the Ghettos where social order collapsed too. Don't ask me to prove it, you will see things that can't be unseen.