The Germans (or at least, the Nazis) had a plan, on paper, to kill everyone in the east and replace them with Germans. Hitler was a Malthusian who believed populations would grow until resources were exhausted, he viewed non-Germans as competition for those resources, and he thought birth control and abortion would reduce genetic competition and make the German race weak (like the French...). So the only solution was to kill everyone who wasn't German.
Here's a quote from Himmler's wiki page: "It is a question of existence, thus it will be a racial struggle of pitiless severity, in the course of which 20 to 30 million Slavs and Jews will perish through military actions and crises of food supply." This was all planned out, and it was going to be on top of the deaths that actually occurred.