Well, for Putin and Russians NATO missiles close to Russian border is the biggest nightmare. You see, russians speak about America every day. They believe in things like Merkel and Macron are american henchmen. And more and more. And Ukraine is like Russian territory which USA has stolen from them. So Russians do believe that if they won't return the Ukraine, the US would soon attack them.
Putin is a cold war product, former KGB agent. So American weapons on former USSR territory is pretty much personal. It's as paranoid as US was about communists, even worse. Their world is still bipolar, so they aren't at war with ukrainians really. They truly believe they are fighting against USA