I don't work in the field and don't know exactly how I'd personally feel about it, but I can see a very strong argument that even one of the most heinous weapons every deployed on Earth likely saved a net million-plus lives.

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...)