No, we don't need to update the definition to retroactively legitimize the mass murder of civilians in warfare. The people killed by the atomic bombs were innocent under any sane definition of the term, and the cruelty behind that was intentional. The US wanted to impart to the Japanese that the violent suffering and death of every man, woman and child would be the cost of anything less than their immediate and unconditional surrender.
Nukes bad, mmkay. Would you have preferred to learn that lesson 6 years later in Korea at the hundred-kiloton level, or 25 years later in Vietnam at the megaton level?
And would you have preferred that Japan be left to the mercies of the Soviets, turning them into what North Korea and Cuba are today?
Childish naïveté, world wars, and postwar geopolitics don't mix.