Why does Stimson feel the use of the bomb was a terrible thing to do?

In the foregoing pages I have tried to give an accurate account of my own personal observations of the circumstances which led up to the use of the atomic bomb and the reasons which underlay our use of it. To me they have always seemed compelling and clear, and I cannot see how any person vested with such responsibilities as mine could have taken any other course or given any other advice to his chiefs.