Hitler had this list of peoples to exterminate:

  * Jews
  * Romanies ("gypsies")
  * Slavs (Poles, Russians, Ukrainians, Czechs, modern Macedonians, Serbs, Slovaks, Bulgarians, Slovenians...)

Jews took the biggest blow, but Hitler didn't plan to stop there. And not all of it was even ethnic. For example, he hated homosexuals and disabled people. He was going to clean the race. Communists were a target, too.

The reason he didn't exterminate everyone at the same time is that he needed slaves and collaborators in the transitional period. People down the list could be used to exterminate those higher up (indirectly, but sometimes directly).

https://en.wikipedia.org/wiki/Lebensraum

As well as almost any Polish professionals: Teachers, professors, lawyers, government officials, religious figures. The goal of the Germans was to dismantle almost all of Polish society and replace it with German society.

The Soviets also had the goal of exterminating the Polish professional class, hence their massacres at Katyn & elsewhere of, among others, Polish Boy Scouts.

Few people realise that the Soviet Union took roughly one-third of Poland when it & Germany invaded Poland at the beginning of the Second World War — and it kept that territory after the war. Polish society in those conquered territories was replaced with Russian society.

This is the point of view taught in Polish schools, so it's unthinkable for a Pole to think otherwise. However, Lithuanians, Belorussians and Ukrainians would beg to disagree.

For me the most fascinating is the case of Lithuania. Polish children are taught about the Commonwealth, common history, fighting together... So when they hear about anti-Polish sentiment there, it makes no sense to them. People are surprised and can't understand it. That's the fault of presenting a one-sided version of history and completely ignoring the POV of your neighbors.

I'm no expert on history, but this is where Nazi ideology seems contradict itself for me:

  * They claimed that all development in Russia was led by people of German (Aryan) origin,
  * Yet they focused their efforts on exterminating the social elite first, to make the society of Untermensh (slave race) fall faster.

Could someone chime in?

I don't see the contradiction. They didn't exterminate people of German descent in the conquered territories, rather they were supposed to become the new elite.

Focusing on exterminating of social elite (doctors, lawyers, famous artists, officers, writers, journalists...) is admitting there was an elite. How can a slave race have people creating culture and order?

You cant challenge Nazi racial ideology as if it was a logically consistent system. It was pretty opportunistic. For example Slavs are technically Aryan (given the theory behind the concept of an Aryan race), but nevertheless the Nazis considered them subhumans because it justified the lebensraum politics. On the other hand, the Japanese was elevated to "honorable Aryans" when they became allies.