Is killing civilians a fair/justifiable act?

I think Hitlers bombing of London, and the death of thousands of civilians was an act of war, and is hard to determine whether or not it is fair. I think during a time of war anything that can help propel your nation to victory should be used. Hitler was at war with Britain, so therefore I think that his destruction of London was justifiable. This event may have even inspired the British people and invested them in the war more than they would have been without the attack, and weakened the German spirits.