In fairness Jack, I think one can say that Bomber Command were the only substantial means of hitting at the machinery of the Nazi war effort. Others were fighting after Dunkirk, e.g. in North Africa or commando raids on the European mainland (Operation Collar was in June '40). The role of the Merchant Navy and the RN surface fleet in sustaining the UK war effort and fighting the U-Boats shouldn't be forgotten either. (Personal interest to declare: my late father served under "Johnnie" Walker's command.)
What "Butch" Harris and his aircrews did was perfectly legal. It is, yet again, a case of applying laws and standards that did not apply at the time. Like any good service(wo)man he was following the legal orders of his superiors and government. I am personally very thankful he did. (I will avoid any discussion on slavery as I would go on for pages.)
The law of war covering aerial bombardment in WW2 was the XIVth Declaration of the 1907 Hague Convention which was in force by default (and still is) as the Third Peace Conference never took place. It in effect only covered war between the British Empire and the USA as France, Germany, Italy, Japan and Russia, did not sign or ratify it. Austria-Hungary signed but did not ratify it. It has been said that perceived advantages from developments in the means of "aerial navigation" had put governments off restricting their use. I am pretty certain tHere were no laws of war covering civilians in unoccupied territory.
The Contracting Powers agree to prohibit, for a period extending to the close of the Third Peace Conference, the discharge of projectiles and explosives from balloons or by other new methods of a similar nature. The present Declaration is only binding on the Contracting Powers in case of war between two or more of them. It shall cease to be binding from the time when, in a war between the Contracting Powers, one of the belligerents is joined by a non-Contracting Power. (my emphasis)
Declaration (XIV) Prohibiting the Discharge of Projectiles and Explosives from Balloons. The Hague, 18 October 1907