It is a legitimate concern but his territorial aggression toward Ukraine in the Crimea and Eastern Ukraine also makes a good case for Ukraine wanting to join NATO. Ukraine has never presented a threat to Russia. Putin is a real threat to the sovereignty of many former Soviet republics.
Yes, Ukraine has never posed a threat, but NATO has, in Putin’s opinion. He’s watch as NATO has expanded and moved closer and closer to his borders, all while Russia has not been aggressive. That changed in 2014. NATO was created to counter the Soviet Union. The Soviet Union collapsed in 1991, yet NATO remained in place and expanded significantly even when Russia was insignificant.