Every time I read something about WW2 I get the feeling that the war was much more about stopping Germany's territorial expansion than stopping Hitler's genocide.
The reason you read that the war was about stopping Germany's expansion rather than the genocide is that the war was about stopping German expansion, and nothing to do with the genocide.
Although the leadership of Britain and America were aware of the existence of death camps, the issue was ignored as the camps held no military importance. The mindset of the western leaders was that the best way to stop the genocide was to end Germany's control of Europe.