This is purpose of NATO - to prevent the lines on the map from changing. Ukraine may not be a member of NATO officially but it doesn't need to be - the Soviet era reason for Ukraine is the same as the NATO, a buffer state.

Russian aggression reinforced the need for a buffer state - before it wasn't obvious, now it is. NATO is intending to force Russia to leave Ukraine and they are willing to play a very long game bc it's a buffer state in play, not a NATO state.

The end result of this strategy is either Russia breaks or the war escalates.

The USSR asked to join NATO, which was quite funny to learn about.

Russia-NATO relations were pretty good, even heading (slowly) towards such membership under Putin in 2000.

Unfortunately, Putin didn't like GWB leaving the ABM treaty, and did like having the old USSR back, so aggression and all else as you say.

Perhaps, he saw the result of NATO aggression against Serbia?

"Russian paratroopers seized Slatina airport to become the first peacekeeping force in the war zone" - https://en.wikipedia.org/wiki/NATO_bombing_of_Yugoslavia

Which was a surprise at the time, and left some convinced it was a UN rather than NATO operation.