There's an argument that the original atomic bomb was necessary in the event the Axis powers could not be defeated by conventional means, but I consider the decision to develop the fusion bomb among the worst indictments of humankind's self-destructive nature. The purpose of a fusion bomb is not historically contingent on winning a specific war, it's specifically a technology to credibly threaten genocide on an unprecedented scale. Humanity could have called it quits with Nagasaki when it came to weapons that destructive rather than taken it as an invitation to develop the technology further in my opinion.

Like all forms of evil once invited into your house it does not leave at your say-so, another profoundly negative feature of nuclear warfare is that now we've invented it unilateral disarmament will likely always be strategically impossible.