If Hitler was killed in 1940, before the Blitz gave Churchill his excuse for the air war, then the Germans could have maintained the sub blockade of Britain to counter the naval blockade of Germany. It would have been hard for the war to continue without the Germans attempting to take over the British fleet. Say, Italy finds out about the death of Hitler and doesn't get into the war. Then France makes an armstice, the Germans sign a better armistice from the French view (you know, Alsace and Lorraine), the German generals unofficially blame everything on Hitler and sign easy armistices with the Dutch, the Belgians, the Czechs, etc. They still keep Poland. They make the French pay a reparation disguised as buying out the proGermans in Alsace and Lorraine at say, twice the assessed value. That gets them enough people to move to Germany to save face for the revanchist Nazis.
Britain has a problem. They can threaten to blockade France and Spain and Italy to keep the blockade up against Germany, but there is a limit to how much the French, the Spanish, and the Italians will put up with. Fighting Germany is popular. Fighting everybody in Europe is not going to be popular.
Then the fact remains that the Russians want the Baltics and eastern Rumania, like they already took by July of 1940. So the trade in population is made. Russia gets the Poles, the Balkans get the eastern Rumanians, etc. Probably settle them in transylvania and crowd out the Rumanians.
And it all quiets down.
Britain and Germany get an armistice after a few months of sub wars. No urban bombing. The POWs go home. No trade relations between Britain and Germany, but the Europeans don't care. The Fascist puppet governments assign voting rights to areas instead of populations and use the conservative rural areas to maintain control of the cities. Lots of European socialist types wind up in the Commonwealth.