Not even touching the political aspect (Ukrainian nationalism being clearly anti-Russian long before 2014), for the Russian military having well established NATO military bases in Ukraine and Georgia is many times worse than for US to have Russian bases in Cuba and elsewhere in Latin America (and you should remember how US reacted to the possibility of those). And no, the Russian military can not responsibly believe in "defensiveness" of NATO for a number of good reasons.

The invasion itself is clearly a bad and desperate move, but it comes after all the previous moves (from trying to be friends with NATO during the pre-Munich years and keeping Ukraine on the hook using the Minsk agreements) were exhausted and it has piled on top of the utter failure of Russian policy on the Ukrainian front since 91 (targeting oligarchs instead of population).

If Russia managed to control Ukraine, even at terrible but temporary cost in 100 years Russia would be much better off with Ukraine than without. It would make Russia even more serious player on so many markets.

They never drew their economic power from people. They could exterminate all Ukrainians and still come on top economically in the long term if they controlled the land.