Holocaust victims
The Nazis killed millions of Slavic people during The Holocaust.  These people included Poles, Russians, Serbs, Ukrainians, Czechs, Bosniaks, and Sorbs.  The Nazis thought Slavs were an inferior race.  However, they also wanted to clear out the people in the Slavic countries, so there would be plenty of Lebensraum ("living space") for "Aryan" people.