At the beginning of the war both sides tried to avoid civilian casualties and only attacked military targets. A single German bomber accidentally bombed London, which caused England to bomb Berlin, which sparked the Blitz as revenge. https://web.archive.org/web/20230109101126/https://www.telegraph.co.uk/news/worldnews/europe/germany/1413593/The-Luftwaffe-blunder-that-started-five-years-of-destruction.html

It not really anyone's fault that terror bombing became the standard for the war, although I've never seen anything that claimed Luftwaffe strategists were enamored with Giulio Douhet and his theories of destroying a nation's will to fight by terrorizing their civilian populace the same way the US Army Air Corps was. Indeed, the Luftwaffe's bomber force consisted entirely of light and medium bombers originally intended to tactically support the army.