This is an existential issue to Russia, they won't regret defending vital interests. It is the west who will regret pushing them on this. If NATO bands together to support Ukraine militarily then it is basically midnight on the nuclear clock. Listen to what Putin said about this point:
"Now a few important, very important words for those who may be tempted to intervene in ongoing events. Whoever tries to hinder us, and even more so to create threats for our country, for our people, should know that Russia’s response will be immediate and will lead you to such consequences that you have never experienced in your history. We are ready for any development of events. All necessary decisions in this regard have been made. I hope that I will be heard."