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.