Russia have no legitimate security concerns.
Ukraine gave up their nukes along with Kazakhstan and Belarus in the 1990s after they signed the Budapest memorandum along with the US, UK, and Russia, in return for the other nations respecting their borders... guess which country broke the agreement.
The Eastern European countries that joined NATO chose to do so themselves, and were free to do so in line with the Helsinki accord, which the SU also signed in the 70s. There was never any agreement made with Russia that guaranteed NATO wouldn't expand. Those countries joined NATO peacefully. What did Russia do when Hungary wanted out of the Warsaw pact?
What is it with far left tankies who bend over backwards to criticise western (primarily US and UK) imperialism, but then make all the excuses in the world for the atrocities of fascist dictators like Putin, Asaad,, and commie dictators like Ping, Chavez, and Castro?