Russia is still part of a small club of countries that can send human civilization back to the Stone Age.

This was indeed quite irresponsible of Ukraine, as we can see: they are now at war with Russia without any real support from the side they were negotiating with (USA / NATO have not and will not deploy to protect Ukraine). The losses (ref. the refugee count, loss of territory) have already been deeply damaging to Ukraine and its future: they were already aging, female dominated population prior to this with very bad economy. I cannot call this behavior responsible in any interpretation; and if these are sovereign actors then the armed veto is legitimate tool in order to pursue national interests.

> Lithuania, Latvia and Estonia are all NATO members (that happened under Putin's first stretch as president).

I think the total number of tanks Baltic states have is around 0; Russia has something like 12,000+. Ukraine has the second largest army in Europe after Russia. Russian govt. probably considers (at this point) it a mistake to not intervene in the Baltic NATO process though.

> If Russia wanted to maintain a NATO-free buffer around its borders, what lesson do you think attacking the country that isn't a NATO member sends to the others?

From the point of view of Russian strategical behavior & deterrence something had to change in the way Russian govt. behaves to stop the further expansion into Ukraine. Clearly, whatever calculus they used previously for strategical deterrence was not enough to stop the expansion (hence, the expansion towards Russia over time).