Russia have legitimate concerns , well they always had those even when they took chunk of Finland lands or when they annexed half of Poland with the help of Nazi Germany.Bro, Russia had legitimate concerns. It is like saying that Iran should never invade Israel or Bahrain for your mentioned reason. We have to, because they are compromising on our national security. By your logic our operation in Syria was a mistake, be careful of what you wish for. What would you do if you see someone is piling up TNT near your house? If you had the power, then you would have beaten the hell of the guy for putting lives of your family in danger. That's all about Russian Op in Ukraine.
but the only things they manage to do was nearing their border closer to NATO borders.
interestingly the area that Russia have concern about , are where Ukraine mineral wealth is coming from . and also build land route to Crimea .
to me Russia solutions for its legitimate concerns always were against UN charter, and our strategy of never support separatist and change of borders by force.