I think saying "NATO is expanding" you mean that Ukrainian choice of signing agreements with EU left Russia no choice.

From Ukrainian point of view, they are an independent country - which may, at different times, get into different unions with different partners.

So, Russian (actually, Russian government) point of view that any Ukrainian motion towards EU signals a threat to Russia doesn't justify Russian hostilities, because Ukraine is an independent country. And when Russia in turn decided to protect its perceived interests, it went against that independence - and all things associated with that.