I think Wellerstein reads more into what Oppenheimer said (years later) about what he thought when he saw the explosion than what he may have meant. My take is that Oppenheimer was saying the quote from the Bhagvad Geeta accurately described his thoughts at the time. That with the atom-bomb we (the United States) have (the power to) become the destroyer of the whole world. I think what he said about Vishnu and Arjuna was to merely provide the context of the quote. He did not mean imply anything about his own duty. This is borne out by what he says next: “I suppose we all thought that, one way or another”. No matter what their personal motivation may be for working on the bomb, they must all have sensed the enormous destructive power of the bomb.