> multiple other manhattan project people had already told him he was going to die if he kept doing it

To me it seems quite reasonable that the people hired

to work on a bomb intended to kill 150,000 people in one go

against the backdrop of a war where 70-85 million died

might not place the greatest value on health and safety, and the sanctity of human life.

On other human lives. They'd probably damn well care about their lives; otherwise, they wouldn't even make an effort to create "better weapons".