According to Richard Feynman, who was one of the physicists to have worked on the bomb, most of the scientists on the project didn't really consider that until some while after the detonation had passed. He explained his sentiments about it in, "Surely You're Joking, Mr. Feynman" by claiming all of the scientists were so wrapped up in this project, these series of continually more difficult problems, that many of them were simply excited to see the bomb succeed. A success, of course, meant that the work had been done well.

He said that it wasn't until later on that he really understood what it was that he had helped create. Specifically, from the book:

I returned to civilization shortly after that and went to Cornell to teach, and my first impression was a very strange one. I can't understand it any more, but I felt very strongly then. I sat in a restaurant in New York, for example, and I looked out at the buildings and I began to think, you know, about how much the radius of the Hiroshima bomb damage was and so forth... How far from here was 34th street?... All those buildings, all smashed — and so on. And I would see people building a bridge, or they'd be making a new road, and I thought, they're crazy, they just don't understand, they don't understand. Why are they making new things? It's so useless.

What's interesting to your point is that almost all of the top scientists who worked on the bomb were among those who later advised against using them.