Considering that Putin came to power in Russia's first (and possibly only) democratic election, I'd say a lot hinged on his view of how the West would treat them. Sentiment at the time was that he enjoyed an unfair media and power advantage due to his former position within the government as Prime Minister and then acting President but it was still a remarkably open election for a formerly autocratic state. From reading his rationalization of his actions in Ukraine, it seems to me that he is motivated by his perception of the US and NATO as an ever encroaching threat to Russia. He likely believes he is effectively a war time president and sees it as his duty to remain in office until he has secured his country against her enemies.

These are the sections of the address that back up this view:

"they are trying to convince us over and over again that NATO is a peace-loving and purely defensive alliance that poses no threat to Russia. Again, they want us to take their word for it. But we are well aware of the real value of these words. In 1990, when German unification was discussed, the United States promised the Soviet leadership that NATO jurisdiction or military presence will not expand one inch to the east and that the unification of Germany will not lead to the spread of NATO's military organisation to the east. This is a quote."

The quote referenced is by U.S. Secretary of State James Baker speaking to Mikhail Gorbachev on February 9, 1990

https://nsarchive.gwu.edu/briefing-book/russia-programs/2017...

"There can be only one answer – this is not about our political regime or anything like that. They just do not need a big and independent country like Russia around."

"its military infrastructure has reached Russia’s borders."

"positioning areas for interceptor missiles are being established in Romania and Poland as part of the US project to create a global missile defence system. It is common knowledge that the launchers deployed there can be used for Tomahawk cruise missiles – offensive strike systems."

He is referring to the Aegis system https://en.wikipedia.org/wiki/Aegis_Ballistic_Missile_Defens...
He is correct that Tomahawk land-attack cruise missiles can be part of that system https://en.wikipedia.org/wiki/Aegis_Combat_System

"It is like a knife to the throat."

No one can know if history would have played out differently after that meeting in 2000 if he had been assured that Russia could join NATO if it worked towards meeting the requirements. We also cannot know his true motivations, perhaps all of this talk of the West as a threat to Russia is simply a ploy to justify their actions. His statements are consistent with reality though and so this could actually be the way he feels about the relationship between Russia and NATO. If that is true, then his actions in Ukraine would be motivated by a now or never viewpoint. If he does nothing and Ukraine eventually joins NATO, they will have two fronts placed very close to Moscow and Belarus will be held in a pincer rendering it next to useless. His gambit seems to be to secure the breakaway republics as a buffer and perhaps extend them to the Dnieper or even further creating a more advantageous position when Crimea, Moldova, and Belarus are considered.