Getting even for the Japanese bombing of Pearl Harbor was not an important consideration by the time the U.S. dropped the atomic bombs on Hiroshima and Nagasaki. They were already devastating Japanese cities with conventional bombing raids and would have completely destroyed all Japan's major cities without using atomic bombs. Once they had captured Okinawa they were within easy bombing range of Japan and could have continued massive air raids indefinitely, just as they were doing in Germany.
Japanese cities were far more vulnerable to explosive and incendiary bombs than European cities because most of the buildings were made of wood. The houses typically had interior walls made of wood and paper. The floors were covered with dried grass matting. The roofs were thatched with straw.
Showing the Soviet Union what we were capable of was probably not a strong consideration at the time, since, for one thing, we didn't really know what we were capable of, and, for another thing, we were not on hostile terms with the Soviet Union. We were encouraging them to come into the war against Japan, and we didn't know what their plans were for the years following World War II.
The most important consideration--and it wasn't entirely Truman's thinking but that of his many advisers--was that it would end the war quickly and save as many as a million American lives. It would give the Japanese an opportunity to "save face," even though they had to accept the demand for "unconditional surrender." Proof that Truman, and his advisers, were right was the fact that Japan surrendered almost immediately aboard the U.S.S. Missouri in Tokyo harbor. The surrender not only saved a million or more American lives but saved even more Japanese lives.