population of Eastern Germany was Slavic not so deep in past
Only the Sorbian population (app 150.000) are Slavic, the rest is Germanic. Saxons, for example, are not Slavic. This is within the current borders of Germany.
Shell i remind you of total WWI casualties? Taking in consideration all sides involved in conflict in Europe, erased population was almost completely of Slavic origin.
About half of the casualties of WW2 were of Slavic descent (20 mln Russians, 3 mln Poles, 1 mln Yugoslavs and others), the rest was of non-Slavic descent.
But you're right in one point: the conflict in Eastern Europe was the heart of the conflict, this was race against race, one totalitarian régime against another. Has of course to do with the racial politics of Hilter cs.