This is exactly it. Putin and most Russians consider Ukrainians to be Russians (and in particular, lower class Russians they can step on). If Ukraine is allowed

This is exactly it. Putin and most Russians consider Ukrainians to be Russians (and in particular, lower class Russians they can step on). If Ukraine is allowed to increase trade with the EU, which will lead to lessening corruption and potentially eventually joining the EU, its economy will develop quickly, and Russians will start to ask why the same can't happen for them. This is bad for Putin, who relies on corruption. Putin will keep them in poverty to make Russia the best place for Russians by comparison. Mearsheimer is kidding himself if he believes an unaligned Ukraine (remember, Ukraine is unaligned right now, and several NATO member states are on the record unambiguously opposing Ukrainian membership, which requires unanimous approval) would be allowed to prosper. Any NATO justification for Putin's actions holds no water. Pretetending that the goal is to demilitarize Ukraine as Putin is doing now is also nonsense because Ukraine's military was never threatening Russia. Putin doesn't care if his puppet government lasts or not as long as Ukraine remains poor.