Ukraine

If those Eastern European states didn't join NATO, they would have ended up dysfunctional kleptocratic autocracies that were effectively client states of Russia, like Belarus. And thats the best case. The worst case you would have had repeats of what happened in Chechnya. The whole notion they could have remained "neutral" was a pipe dream

Russia had a chance to reform and join the West but they chose the path of rebuilding regional hegemony

Also I would note, the military spending of NATO, at least in Europe, nosedived after the Cold War ended. After 9/11 there was a bit of a bump, but not much. So how does that align with the notion of them being some threat to Russia?
Sadly, some of them have turned into dysfunctional, kleptocratic autocracies like Hungary, or have flirted with it (Moldova, Romania) or have very flawed democracies like Poland while Serbia remains a staunch pro-Russian ally due to the disintegration of Yugoslavia and 1990's Balkan Wars in spite of NATO, E.U. or American economic and military support. Many of these above-mentioned countries have had to endure centuries of direct/indirect Russian control, hegemony, invasion, Kremlin-backed coups, failed uprisings, and nationalist movements and those ties and contacts aren't easily broken.

Russia doesn't want to really reform and adopt a more 21st-century post-imperialist International cooperative stance of collaboration because due to it long being a economic and cultural backwater (seldom wasn't officially abolished until 1861) that didnt begin heavily industrializing until Stalin's brutal 5-year plans of the 1930's, plus they were mostly cut off and culturally isolated for centuries, their overall foreign policy mindset remains stuck somewhere in the mid-late 19th century phase. Given that their one of the world's largest countries and still formidable Soviet-era nuclear arsenal, that's an instant recipe for constant worry, suspicion and fears of geopoliitical bullying, threats and invasions.