What's happening is essentially the struggle of freedom against control and oppression. The major part of global history since the year dot has always boiled down down to this.
The only security threat that a free and independent Ukraine poses is the threat posed to Putin's gangster-state regime by the existence - and example to the Russian people - of democracy and freedom.
The above also applies to NATO - a defensive alliance of free and democratic nations. The idea that NATO would ever invade Russia is laughable, all the more so given Russia's nuclear weapons.
Putin fears the spread of freedom and democracy into Russia. Everything else is lies from a thug dictator who, even a week ago, denied that Russian troops would invade Ukraine (beyond being already in Crimea).
There are credible people (e.g. from Bellingcat) who say that Russia cannot finance this large-scale military invasion for more than another 2 - 3 weeks.
I therefore think, if Kiev is still in Ukranian hands after this time, that the risk of Putin using nuclear weapons is, unfortunately, significantly higher than most people suppose. I believe that he might well use a tactical nuke in Ukraine if that becomes his only way of avoiding military humiliation.
What NATO's response would be, if he does use a tactical nuke, is anyone's guess.
Finally, please note: I'm here to discuss the Ukrainian situation - nothing else.