I don’t know if NATO would respond to a nuclear attack on Kyiv. Would NATO attack knowing the destruction of London/Paris/NYC/etc would be almost guaranteed?
First of all, if Putin wanted to level Kiev he could do it with conventional bombs and artillery. Nuclear weapons are not needed.
But second of all, *why* would he want to flatten Kiev? Why don’t you read his goals in his own words? https://web.archive.org/web/20220224024154/http://en.kremlin...
He says he sees Russians and Ukrainians as one people and wants to unify them. No, I’m not defending him, I oppose war in all circumstances.
But it’s important to understand that he’s not some evil maniac out of a comic book that wants to kill everybody, he has actual goals.
And besides taking pieces from your neighboring country, waging long shadow war in Donbas, telling everyone how Ukraine is not really a country anyway and then invading doesn't really seem such a great plan for unifying nations.
As far as I understand relations before Russians and Ukrainians were pretty OK before 2014 events. With his own actions Putin has made a country that has roughly 1/3 of the population of Russia mostly against him. Healing these wounds and relationships will take decades even once the war ends.