Because the Nazis held that, as the Master Race chosen by the Will of History, they had the moral right to conquer any territory holding natural resources the Fuehrer deemed necessary to sustain the Folkish State, enslaving any people living there who were deemed useful, and either expelling or exterminating the rest. The Germans killed between 11 and 13 million civilians as part of their campaign to cleanse their Lebensraum. When Soviet troops rolled westward after Operation Bagration, they found countless towns and villages that had been completely depopulated by the Nazis, some of them resettled with Germans. In some case, the inhabitants had been moved to slave camps, and in others, they had been murdered.
I don't understand why modern Nazi sympathizers don't seem to have any awareness of the Lebensraum program at all, let alone how foundational slavery and genocide were to it. It's like being a hard core internet tradcath who's never heard about papal infallibility. They seem to think Hitler was a liberal with nationalistic tendencies who was just a bit too based for his own day.