I am glad that you understand. And Jews themselves had many willing colaborators. The problem is not with the Poles, or with the Jews or with any particular nation. The problem is with the fallen human nature which is shared by all.
One of the mechanisms of vilification/dehumanization of some group is projecting the universal tendency toward evil on the selected group. This was used against Jews by the Nazis, it was used by the West against the Serbs. But the border between good and evil goes through the every heart. As Solzhenitsyn said:
"It was granted to me to carry away from my prison years on my bent back, which nearly broke beneath its load, this essential experience: how a human being becomes evil and how good. In the intoxication of youthful successes I had felt myself to be infallible, and I was therefore cruel. In the surfeit of power I was a murderer and an oppressor. In my most evil moments I was convinced that I was doing good, and I was well supplied with systematic arguments.
It was only when I lay there on rotting prison straw that I sensed within myself the first stirrings of good. Gradually it was disclosed to me that the line separating good and evil passes not through states, nor between classes, nor between political parties either, but right through every human heart, and through all human hearts. This line shifts. Inside us, it oscillates with the years. Even within hearts overwhelmed by evil, one small bridgehead of good is retained; and even in the best of all hearts, there remains a small corner of evil."
(Alexander Solzhenitsyn, Gulag Archipelago )
If someone wanted to be a hero, at the time of WWII he would protect the Jews, if someone wanted to be fair in 1999 he would raise the voice to defend the Serbs (heroism was not needed in 1999, little civil courage and honesty was enough)