Atomic bombs slaughter all people in a region. Most of them are not soldiers. Wars in old times were fought among soldiers or samurais. There was also the way to battle, which was fought exclusively man-to-man by generals of both sides.
However atomic bombs had burnt all different kinds of life to death, including women, children, and infants. Such weapons should not be justified.
Even for the war between countries, there are rules, such as "International Humanitarian Law", or "The international jus cogens.. The act of dropping atomic bombs definitely goes against these rules. We, as humans, and as human kind, must not forgive weapons that kill people whoever they are.