To start off the scientist initially working in "Fat Man and Little Boy" were working their hardest to build such a destructive weapon such as the atomic bomb under the influence that their enemies also had the technology to build it. With that assumption building a bomb with that much devastative power seems justified with the intent to use it as a countermeasure to protect their own country. It wasn't revealed till much later in the project that their enemies could not build nuclear weapons and thus the scientist started to hesitate to unleash such a weapon when there was no threat of it happening to them. Compared to "Gojira" where the main Japanese scientist who created the oxygen destroyer went through the whole movie not unleashing his weapon even though for the greater good of his country he could not expose it for then other organizations could copy his weapon and it would cause much mass destruction. These two parallel because of when both scientist realize there country is at a complete threat of annihilation they do whatever they can to prevent it no matter the cost. In the end the main scientist in "Gojira" used his weapon to defeat Godzilla to save his country. To think about the morality of the US scientist who ended up dropping nuclear weapons on Japan they justified saving millions of lives of their own soldiers that would have died to attack Japan.