The 2 atomic bombs, the shock it created in the political leadership of Japan, served to quicken the end of the war against Japan. If they were not used, the Allied will likely have to invade the main islands of Japan. To do so, just like Europe, the invasion will be preceeded by intense strategic bombing. If we use the fire bombing of Dresden and the estimated 25K dead, as an example, the numbers are clear, a conventional war will cause many more lives than the death from the atomic bombs.
The atomic bombs were thought to be the most expediant way to end the war, and they did end the war.