If you read that whole letter, the tone taken was one of the scientists wanting to give Japan a chance to surrender before being nuked. They had little compunction against the actual act but rather wanted it to be an act after other options were exhausted.
The US military opinion at the time was it would save lives and hasten the end of the war. Japan had had opportunities to surrender and showed no signs of wanting to do so. Truman sided with that opinion.
The bombs of the period, in the kiloton range, resulted in about the same level of destruction on a city that a large scale conventional bombing raid did. A raid on Tokyo with conventional bombers resulted in more deaths and wider destruction than either atomic bomb. But it required several hundred bombers and thousands of aircrew to pull off. Nuclear bombs of the sort then available were a better option for roughly the same level of destruction.
It would be thermonuclear (fusion) bombs that were truly terrifying.
https://nuclearsecrecy.com/nukemap/
Try the results using a Hiroshima bomb versus something in the MegaTon (MT) range on this.