I can’t answer for any scientist on the Manhatten project specifically, but Albert Einstein was, although he sent a letter to Roosevelt urging them to start the development in fear of the germans, highly against using nuclear weapons and didn’t want to help creating it. After the war he wanted to stop it.

A lot of scientist shared the same fear - that the germans would produce it before the allies, and therefor supported it. Scientist generally understood how dangerous this was, and had no interest in producing this for gains in a war. We know a lot of scientist looked down on how science was used to create gas during WW1, which definitely shaped their view on WW2 and the Manhatten project.