The bombs were extremely effective and showed the Japanese our immense capacity for devastation. The pain involved with a death associated with an atomic blast can be considered negligeble...the people had no time to really suffer. It was instantaneous. The Japanese citizens died yes, but the huge majority of them did not suffer a painful death.
We showed mercy in our killing...they were extremely savage and brutal (ie read up on the immensely cruel medical testing they did on POWs...it was worse than the Germans and also dont forget how they treated the civilians of conquered lands like the phillipines)
Its already been mentioned previously, but a land invasion wouldve been MUCH worse.
Wikipedia
* A study done for Secretary of War Henry Stimson's staff by William Shockley estimated that conquering Japan would cost 1.7–4 million American casualties, including 500,000–1,000,000 fatalities, and five to ten million Japanese fatalities. The key assumption was large-scale participation by civilians in the defense of Japan.
For context, the Battle of Normandy had cost 63,000 casualties in the first 48 days; and the Battle of Okinawa ran up 72,000 casualties over about 82 days, of whom 18,900 were killed or missing. Several thousand soldiers who died indirectly whether because of wounds or other causes at a later date are not included. The entire war cost the United States a total of just over a million casualties, with 400,000 fatalities.
How many American casualties did we have with dropping with the bomb? A hell of a lot less than 1.7-4,000,000!
How many Japanese died in the attack? ...about 250,000 but thats a hell of a lot less than 10 million. The Japanese government convinced its civilians that we wanted all of them dead and would rather jump off a cliff (after throwing their kids off the cliff) before accepting occupation by us.