I for one am very glad of the lesson taught to the world by the detonation of two bombs during wartime Hiroshima and Nagasaki. I live in Australia, and we were next on the invasion list. I regret the loss of civilian life that the Japanese people suffered as a consequence of war and I am glad that Japan is now one of our greatest friends and highly respected.
I think it was inevitable that we would discover some method for extremely large scale destruction and I'm glad that humanity has so far not used the technology since WWII for anything more serious than sabre rattling- and that's bad enough. We need to understand how to deal with this kind of power because as much as we may not like it, it exists and it has existed all along. There are worse things than nuclear weapons too, so the learning experience is useful in other areas.
I personally support the idea of a monument to those men who were part of the Manhattan Project and think its pretty shallow for people to call them murderers etc. Firstly: fuck you- you weren't there (and neither was I). WWII was a response, NOT an invasion.
I agree with jtownatpunk.net 's comments. Take advantage of the learning experience while there are still people alive who can look you in the eye and tell you what it was really like back then. Sadly for the Manhattan Project the time for that is likely past.