No, he should not be held accountable for the nuclear bombs that the United States used in World War 2. He simply found out the science behind the bombs, so that they could be built. He had nothing to do with making the decision to fire the bombs off in Japan.
I do not believe Albert Einstein should be held accountable for the nuclear bombs of World War II, for two reasons. First, Einstein denounced the use of these bombs. Secondly, you can't hold someone accountable when they are dead. Insinuating that he was somehow at fault is not going to change their mind about such a prominent figure head.
Einstein was a scientist. His researches led him to discover the principles of atomic fusion and fission, but he did not set out to create a weapon. The fact that his discoveries could be perverted into bombs and other destructive measures was not his fault, any more than it is the fault of the person who first made fire that fire can burn and destroy.
Albert Einstein was only doing what he thought would best defeat fascism, and later went on to regret his decision. However, even if Einstein hadn't contributed in the way he did, the bombs would have been built anyway, and not even just by the Americans, but the Germans and Japanese were close to making ones as well.
Albert Einstein didn't work on the Manhattan Project. If anyone should be held accountable for the nuclear bombs of World War II, it is Dr. Robert Oppenheimer who headed the project. Einstein wrote to FDR that the Nazis were experimenting with uranium-235 to try to make nuclear fission bombs. Six years after that, the United States successfully dropped two of these devices in Japan. Einstein wasn't responsible for the nukes, the Nazis and the United States were the ones behind the impetus for the project.