The question really hangs on when nuclear energy was thought possible; because once it is thought possible it's easy enough to speculate whether it could be weaponised.
Now, in 1896 Wilhelm Rontgen and Henri Bequerel discovered radioactivity, and then in 1903 Pierre Curie announced that Radium salts radiated heat without cooling down and thus revealing a new source of energy; a year later, Ernest Rutherford showed that alpha particle radiation released enormous amounts of radiation, he wrote:
The discovery of the radioactive elements which in their disintegration liberate enormous amounts of energy increases the possible limit of the duration of life on this planet, and allows the time claimed by the geologist and biologist for the process of evolution.
Here, he was referring to a controversy between Darwin & Kelvin of the age of the earth; Darwin had come up with an estimate of 200 million years by considering the process of erosion on the earths surface, whereas Kelvin had estimated only 30 million years by supposing gravitational contraction powered the sun.
The discovery of radioactivity freed theorists from considering gravity as the source of the suns energy, but it was shown that the sun had very little radioactive elements being mostly composed of hydrogen so some other process was required.
In 1905, Einstein had established his famous equation relating energy and mass; this fueled speculation that this could be the process that fueled the sun. Empirical evidence for such a claim came in 1919 when FW Aston demonstrated that four hydrogen atoms weighed more than a helium atom; then Sir Arthur Eddington, leveraging Einsteins insight, argued that this mass difference was what powered the sun. He said:
If, indeed, the sub-atomic energy in the stars is being freely used to maintain their great furnaces it seems to bring a little nearer fulfillment our dream of controlling this latent power for the well-being of the human race - or it's suicide.
This must be one of the earliest acknowledgements that nuclear energy could be hugely destructive; this it's probable that a debate had been circulating amongst scientists between the discovery of a new energy source by Pierre Curie in 1903 and Eddingtons speech in 1920 to whether such a source could be weaponised.