Well technically, Einstein signed the letter to FDR that encouraged the US to develop the atomic bomb before the germans; though he nothing to do with the project, by signing that letter he was one of the major reasons FDR started the manhaten project. He did later regret it though, and advocate for nuclear disarmament.
@Stormthehouse
You mean Oppenheimer ? Einstein was not very involved with the development of the atomic bomb. His credits to atomic energy are over cited. His theories were what brought about the atomic bomb, not him building or testing devices.