From my point of view it violates the non-aggression principle so it is morally wrong.
Now that's all fine and dandy, but unfortunately states don't act morally they act to maintain and advance their power. The number one priority for any state is to maintain its security. Russia's security without a doubt has been eroded since the start of Eastern NATO expansion in the late 90s and NATO's war against Yugoslavia. For peace to be maintained that erosion of security had to be limited to areas that didn't encroach upon Russia's vital interests. Powerful countries go to war over such interests. Given how weak Russia is economically it can't dominate Europe like it once did, however and it has stated this before, Georgia and Ukraine were where they would draw the line (i.e. they were vital interests). The US's response to this was to intervene in Ukrainian affairs by supporting a coup of a democratically elected president, providing billions in military support, advancing NATO membership aspirations and even had politicians on the ground in Ukraine advocating for the westernisation of the Ukraine. If Russia swallowed this, they would be abandoning the Russians in the Donbas who they committed to support, Russian minorities in Ukraine would continue to be discriminated against, they would face an emboldened Ukraine that would then focus its efforts on re-taking Crimea. Without Crimea they would not be able to have a navy in the Mediterranean. It would also then allow western forces right up to its border and provide the US with a great advantage in terms of their ability to knock out Russia's nuclear arsenal. So yes from that perspective I can see where Russia is coming from wanting to prevent that given it is a powerful country that does not support the liberal agenda of the west and rightfully, I think, fears US world domination. They tried for years to go down the diplomatic route but NATO and the US just laughed in their face.