How does this justify the Allies' bombing of civilians? Just because your enemy does it doesn't suddenly make it right for you to do it as well.
Because those civilians and those cities like it not, supported the war effort. Not only that intermingled in the cities are military personnel and weaponry. Maybe it isn't kosher, but lining people up in front of ditches and shooting them is a step beyond the line of being casualties of war and genocidal maniacs. Starving people to death in the millions, because you invade and annex them, make them slave laborers on their own lands and take the food they grow and send it to Moscow. There is an obvious point at which you cant not deny the true evilness of ones action.