If a body gives off the energy L in the form of radiation, its mass diminishes by L/c²The form "E=mc²" of his insight is a matter of marketing but it is a good marketing. However, it is good that the people who invented this marketing trick didn't present themselves as co-authors. ;-)
Can Einstein be blamed for Hiroshima and Nagasaki? Well, a little bit. Look at this provocative sentence ;-) in his article:
It is not impossible that with bodies whose energy-content is variable to a high degree (e.g. with radium salts) the theory may be successfully put to the test.