'The Germans wanted to kill a city': New documents show Nazis planned total destruction of Warsaw

One of the forms of genocide as per international law is trying to destroy cultures, not just people. The Nazis very clearly believed in also doing that, to make sure that not a trace of their victims would remain.

Yes, in a twisted way it makes some sort of sense. They didn't completely think in terms of descent, they also believed in cultural influences. So, to remove those they thought that they also had to destroy every trace of the people they were murdering.