That's the pitfall these discussions invariably fall into - debating the morality of deploying rather than the wisdom of developing a capacity in the first place. The primary justification for secretly studying the feasibility of the fission chain reaction was the fear that Germany (not Japan) might be developing an atomic bomb of their own (an early form of the subsequent Cold War's 'can't prove a negative' about what weapons the Soviets might be developing).
What about a pure Soviet entry into the war? You have to look at what happened to Poland and Eastern Europe when the Soviet army invaded. Once again, by any metric, this was the best possible outcome at the time.
But it was unquestionably a local maximum - the best solution at the time, with horrific consequences afterward.