This is a difficult question to answer based on the limited knowledge of science at the time. From today's perspective it's easy to see that using the atomic bomb was a terrible decision that negatively impacted the lives of millions as well as wiped out the lives of millions more. However, in the past the side effects of radiation on the surviving populous were not as clear as they are today. So from the perspective of the past it was easier to justify. However, I would not say that made it any more right.