so if your enemy is evil and barbaric it is then morally justified to also behave in such a manner to stop him?

Germany murdered between 6 million and 11 million civilians and POWs in their slave and extermination camps. The Allied bombing campaign against Germany killed about 600,000 German civilians throughout the war. War is not about choosing between good and evil, it's about choosing the action that is less evil to win.

The German army of WW2 was the greatest fighting force of the 20th Century, you do not beat such a thing by being magnanimous, you do it by doing as much as possible to destroy it's infrastructure and war ecomy.

The reason the Blitz failed was because Germany signally failed to attack the three things that make fighting a war possible: industry, power supply, and road and rail networks, and also because it was not carried out in sufficient strenght. The Americans and British realised this and set about to correct it. Unfortunately many of these three targets are located in urban centres, and since there was no such thing as 'precision bombing' it was necessary to use saturation bombing to ensure destruction of these targets.

So now the Soviets don't count as allies, do they?

The war the Western Allies fought was in order to free Poland (and the other nations that were conquered) from German rule. The war the Soviets fought was to repel an invader and to conquer as much territory between Russia and Germany as possible in order to secure it's borders from future German (and capitalist) invasions.