Einstein himself did not actively work on the bomb. The atom bomb was only conceived of after his work was done, but his theories showed others that it was possible. He wrote a letter to FDR urging him to build it, and this is despite having been a pacifist.
Actually the fission bomb really did not need any of his theories to be "discovered". People discovered fissionable material and new it to release energy, and later found the mass defect and that it was consistent with E=MC^2, but E=MC^2 actually works in chemical explosions, its just that the energy released, and mass destroyed is thousands of times less so what I'm saying is that people didn't need to know that E-MC^2 to figure out that a fission chain reaction would release enormous energy, any more than they would need it to know that dynamite would do so.
As for Dynamite, you may know that Alfred Nobel invented it and described it as a potential weapon of such power that it would surely end war forever, and he made his fortune from it which he later left to promote peace.
And the real whopper of E=MC^2 is not fusion or fission, which like chemicals only transform a fraction of matter into energy, but rather matter-antimatter annihilation in which ALL of the matter is turned into energy.