For Putin having Ukraine part of NATO is as dangerous as it was for the US to have Russian missiles in Cuba. It's something he is not willing to risk.
So almost any price is worth it for him.
And his proposal, for Ukraine to be a kind of neutral zone, was not for Putin, it was for the USA and Ukraine.. so that Ukraine could survive as an independent country.
Mearsheimer is West point graduate and air force officer...
His biases are if anything pro USA.
I would agree that makes sense from Puntin's world view. But not for anyone else's including a lot of Russians.
I watched the the start of the video, apparently a country simply being allied with the west and the US is an "existential threat" to Russia.
Ukraine or Georgia are not going to invade Russia after joining NATO, they simply don't want be a part of the next Russian empire dreamt up by a tyrannical regime.
This has now come to pass, a Russian puppet government lost power in Ukraine, Putin saw his power slipping away and invaded.
And NATO is supposed to be the aggressor and the cause of this?
I've found the first 25mins of this conference by professor John Mearshimer https://youtu.be/Nbj1AR_aAcE