A human-like entity appeared out of nowhere and produced miracle after miracle -- displaying immense powers like humanity has never seen before and could never comprehend in their entire existence. Not evil of nature, it transforms the world everywhere she goes. Reviving the dead, unlocking the hidden potential of the country she appears in, terraforming the earth or bringing in supreme knowledge -- people are never the same wherever she passed.
It is so big that the entity is seen as a god by most the population.
But at the same time, and due to the incredible power brought with her, the other countries classify the entity as a threat to the world and humanity. They are terrorized by what she could do. Both because they can not comprehend how she works, and because the entity could probably destroy them any instant if she was displeased.
In a desperate attempt, the governments (most of them) send special forces to try to collect intelligence on the entity. It is more about deciding whether it is a threat, or whether they can live in symbiosis without risking their total extinction. It obviously fails. The mission ends up with the cover being blown up, the entity displeased, and the special forces left with such terror that they shoot it in the head with a 9mm.
You guessed it. The eldritch abomination does not die from the shot and retaliate by killing all the men sent to monitor her every step in a horrific, incomprehensible way... proving at the same time that she represents a potential menace to humanity.
At this point, all governments on earth are aware of what happened. Yet, there is still distinct political forces differentiating them, and it is no different from today's world (apart from the entity's presence). Thus, it would be difficult to have a quick decision from an institution representing Earth.
That is why, it is one nation that freak out on its own, and arbitrarily decides to nuke the entity to avoid the risk of having it begins a massacre.
What could have led this nation to take the radical choice of nuking the eldritch abomination, disregarding humans rights, potentially alienating all the others country in the world, and likely starting a war ?
PS: I know a nuke is exceptionally complex to launch, it is hardly something you just decide to use like that, and needs a long process to start. But I need something of the like in my setup. For it is required that people understand the terror of the device, and how crazy powerful it is. There has to be collateral damage and long-lasting consequences on the world.