"The worst atrocities took place in XVII century and at that time the area was under Polish rule (and Poles were among the victims too)."
The worst atrocities took place during the Holocaust when the Nazis together with their countless collaborators in Eastern and Central Europe murdered millions of Jews.
Just for your information - Poles (followed later by Serbs and Greeks) were the three nations which FIRST challenged Hitler militarily. But Poland alone was too weak to defeat Nazi Germany and you should not blame Poles that they lost!
Poland was the main nation in Europe which allowed mass immigration of Jews and gave them freedom to prosper and develop their culture. Even after Poland weakened, became poorer and less attractive place she should command grattitude.
That is true, but least one forget the 6 million Poles who die in Concentration camps, 22 million Russian civilians, 1 million yugoslavs, 1 million Greeks, 1 million Italians (after it switch sides), 2-4 million French and few million Germans who less then agree with Hitler.