Ukraine has been super militarized with anti-Russian sentiments rising since they illegally stole a part of their country in 2014 and started providing money, arms, vehicles, and soldiers to separatists premised on said separatists murdering their fellow citizens and providing a thin pretext for Russia ultimately taking more of Ukraine.
Given the profoundly destructive nature of any such conflict with Russia and the impossibility of winning or even surviving without a coalition of supporters there is zero chance of Ukraine ever starting a conflict with Russia itself.
Given the risk of nuclear war and the impossibility of pushing Ukraine to start such a conflict there was never any chance of NATO either starting such a conflict OR being able to start one by proxy.
It’s hard to argue that Russia had security concerns when the only person in a position to light this candle is themselves.
NATO was virtually entirely a mutual defense pact vs Russia in their previous incarnation as the USSR. Inducting Russia into NATO would only serve to give them veto power and influence on an org which virtually exists to defend against THEM! It makes no coherent sense nor would it somehow provide the Russians some share of “super profits” it would solely give them an opportunity to undermine NATO which is why Putin wanted it.
The material basis for stealing the Ukrainians country from them and murdering its children is that by doing so they gain access to tax payers, resources, people, strategic resources, land, fossil fuels etc. Based on what we know about their strategic planning we have every reason to believe they thought this would be an inexpensive and quick affair that would be concluded in a matter of days with minimal loss of life.
It is purely a function of avarice, stupidity, and immorality. It is no more complicated than asking why a burglar invaded a home and took the lives of people there when he just ended up leaving bloody himself. They did it because they thought it would profit them and because they thought they could get away with it.