Japanese civilians in battle zones routinely walked up to Allied soldiers with grenades to blow them up. They threw themselves off cliffs rather then surrender.
We still have this concept that somehow the German and Japanese citizens where not involved in the actions of their governments. It's as much of a myth as the myth of a "Clean Wehrmacht"
And about civilians attacking Allied soldiers, do you have references for people in Tokyo, Hiroshima or Nagasaki doing it?
The Battle of Okinawa seems like an example - Both willingly and by coercion, a large portion of the civilian population was sent into the battle to die with minimal training or equipment, including children.
Is there some reason to think the populations of those cities would have acted differently if the front lines came to them at that time?
They literally disassembled their factories in many cases, and put equipment into people's houses. The broke the factories into workshops, which where highly inefficient. The Japanese believed this would make them resistant to bombing, but also scale well since they didn't have to centralize at single sites. (the exact opposite of Fordism). They were incorrect on both counts.
And no, there are no documented records of people in Tokyo, Nagasaki and Hiroshima doing it - killing people tends to make them unable to engage in further violence (though it may encourage societies to further violence). but there are records in Okinawa, and in virtually every other sizable Japanese population engaging in violence.
That even includes in Hawaii. See the Nihau incident.
About the bombings, the targeted bombing wasn't working very well because of weather, so they decided to obliterate cities with incendiary bombs, civilians and all.
But returning to mass murdering civilians to advance military objectives: what would you think of the Soviet Union destroying with atomic weapons Munich and Frankfurt, to force Hitler to an unconditional surrender? Would reducing the deaths of soviet soldiers be a good justification?
Your point is good - that's a isolated event - but it was a isolated event that was taken into account in the military actions that followed.
And yes. If the soviet union had destroyed munich and frankfurt (Frankfurt was the transportation hub of the western front) to force Hitler to surrender it would have been justified. To draw the full conclusion - it would not have been justified to bomb Geneva (since it wasn't a military target).
And frankly, bombing Frankfurt would have been far far less heinous then what the Red Army did to East Prussia, which could not be justified. The difference there is that the soviets had already overrun Prussia when they committed their war atrocities. Likewise, what US solders did to German PoW in more isolated circumstances.