How did Einstein,an ardent pacifist, become the lead researcher and advocate for the development of the atomic bomb?
First, I would argue that the premise of this question is not accurate. Albert Einstein was not the lead researcher by any means in the development of the atomic bomb. He was also not exactly the lead advocate for its development. That said, he was involved in the creation of the bomb to some degree. His involvement was due to his fear that the Nazis in Germany would invent the bomb first.
Einstein was not deeply involved in the research that led to the creation of the atomic bomb. He had no role in the actual workings of the Manhattan Project that created the bomb. His only real role came at 1939, three physicists who were much more involved in research relevant to the atomic bomb came to Einstein and asked for his help in writing a letter and getting it to President Franklin D. Roosevelt. They wanted to warn him of the immense danger that atomic bombs would pose and to tell him that they feared Germany would produce such bombs. They chose Einstein for two reasons. First, he was the most famous physicist in America and would therefore be likely to be able to get a letter to the president. Second, he was a Jewish refugee from Germany and therefore could be counted on to hate Nazism and to worry about what would happen if Hitler got the bomb. It is for these two reasons that Einstein became involved. He helped write the letter and helped get it to Roosevelt, but that is the extent of his involvement with the atomic bomb.