If the goal was to avoid war in Europe, then keeping a buffer of neutral countries between Russia and the EU/NATO would have been the logical course of action, when failing to integrate Russia in the European family.

It seems to me that the EU and US didn’t know when to stop and only looked at the potential economical and respectively geopolitical gains, while not giving enough weight to the risk of conflict when it was known what Russia’s red lines were.

This was a policy blunder. Looking at it from a moral or international law perspective is a dead end when the primary driver of the chain of events leading to today was geopolitics.

You are ignoring the will of the people in a buffer state. In the EU they see prospering liberal democracies and on the other side it's permanent authoritarian kleptocracy, which by the way still sees them as part of a lost empire. Of course they are going to want a path to the EU.

You seem to be assuming that countries may decide on their own freely who they associate with. This is the case in international law, but not in reality, as can be seen in many examples concerning Russia or the US or China.

Realpolitik vs ideal-oriented policy. It wasn’t the buffer states driving the decision, it was the US and NATO. They had to choose between buffer states and expanding NATO. The developments in Ukraine were predictable and likely avoidable had they chosen differently - see the fate of Belarus or Georgia or Ukraine before 2004 and before 2014.