The issue isn't Ukraine becoming "NATO controlled" (all NATO members actually control themselves; NATO is a voluntary alliance), it's about keeping it Ukraine-controlled. Putin wants it to be controlled by Moscow instead, and that's why he invaded.

Besides, there are already NATO members close to Moscow. And you know why they joined NATO? Because Russia is also close to them. Putin-supporters keep arguing that Russia needs security guarantees, but completely ignore the security guarantees of Russia's neighbours. And Russia is clearly a far larger threat to its neighbours than those neighbours are to Russia. Few countries have as much buffer built in as Russia does. What right does Russia have to demand entire countries as additional buffer? Where is Ukraine's buffer?

> If Ukraine had announced permanent neutrality and buffer state status (like Switzerland)

Switzerland is not a buffer state. They're neutral because they choose to be. Chose, because Russian aggression is making even the Swiss consider choosing sides.

> that would have been the best outcome for Russia.

Because then Russia would be able to coerce and invade with impunity. Ukraine doesn't need neutrality, it needs security guarantees. Guarantees that Putin is clearly unwilling to give, and NATO is able to provide.

Putin's aggression is Russia's biggest enemy.