Personally, it should not negatively impact lives. For example, I can't see myself saying "I will take this job for the money, and the gun I help engineered is used to kill humans during combat".
If that is the case, is it moral to refuse to work on the Manhattan Project if asked?
(I also agree that working on the next generation of the atomic bomb or the next revision of the M16/AK47 does not hold the same promise of saving lives, of course...)
Modern society has already made the choice though. We sacrifice a few for the many all the time, and they are called heros and duly celebrated.
Do you know of any text, interviews or discussions with any of the scientists on the Manhattan project on the morality of their work?
It's really an argument that can't be made because the other event never happened. The weapon killed people, that's it. I don't know if there's a moral side at all?