You're running 2 trains of thought that almost, but don't quite, run parralel.
The whole point of nuclear weapons are that they are highly destructive. thats why people would use them, its a hell of a threat. But that is generally a fusion reaction.
As a form of energy, its incredibly efficient, despite the process to acquire nuclear fission being incredibly energy consuming.
There's the why, the ethical questions that they arise afterwards are a different kettle of fish. And people don't always do what is ethical.