I disagree with Truman's decision to drop the bomb, however I think Truman was motivated by anger at Japan rather than fear of the Soviets.
Yes, the bombs killed hundreds of thousands of people, in a war that had already killed some 70 million.
The bombs killed 129,000 and had we invaded Japan the war would have dragged on for perhaps another year and would have claimed another million or more lives ... including 500,000 Americans. Dropping those bombs was the quickest and most humane way to end the war. End of story.