1) At the time the atomic bombs were dropped, Japan still occupied vast parts of China, all of Korea, Indonesia, parts of Burma, Thailand, Vietnam, the Philippines, New Guinea, et al. The final body count will never be known for sure but somewhere between 15 and 30 million people (almost all civilians) in these and other occupied lands died under the Japanese boot. To the point, for every month the war continued, 200,000 more people in these lands would die. The B-29's stopped this.
2) It is hard to exaggerate the fanaticism with which the Japanese fought in the Pacific, particularly in the last 18 months of the war. Very few Japanese prisoners were ever taken. (And the treatment of POW's taken by the Japanese would make Auschwitz camp guards vomit.) Local populations were told lies about the atrocities Americans would commit if they won to keep them from siding against the Japanese. There is film of women with children in their arms jumping off 100-foot cliffs at Saipan to avoid falling into the hands of American soldiers. If only they knew...
3) The climax was Okinawa, where 77,000 Japanese regular army and 23,000 conscripts from the local population had to be killed to the last man. As a result, somewhere between 100,000 and 200,000 Okinawan civilians were killed. This was on top of 12,000 American soldiers, sailors, airmen and Marine KIA, including 5,000 sailors killed and 34 ships sunk by kamikazes, many more ships knocked out of the war, by far the worst beating the US Navy ever took in its history. The final count was therefore 150,000 to 300,000 killed at Okinawa in just 3 months of fighting.
It was with this butcher's bill in his hand and the specter of an invasion of the Japanese home islands becoming another Okinawa times 50 or 100 that Harry Truman made his decision about the atomic bombs. The population of Okinawa at the time of the invasion was about 450,000. Japan's at that time was about 75 million.