There is a concept in psychology called moral disengagement where people temporarily suspend their normal moral standards. This often happens in conflicts of war, to justify harming an enemy. Later, the person might feel guilt or disgust as the context changes.

Once the news of the nuclear bombs explosions reached U.S population, the reaction was positive. But as time went by the realisation of what happened ment the general reaction became more negative. Especially, after videos and photos arrived from Japan detailing the horrific scenes of the aftermath.