Ukraine was never anywhere close to joining NATO before. If anything, this invasion makes a stronger case for them joining NATO now, along with Finland and Sweden.

Putin is scapegoating NATO. From the beginning of his 23 years in power (including the 4 years where he administered the government through Dmitry Medvedev), Putin has dreamed of rebuilding the Russian Empire. In a speech in 2005, he called the dissolution of the USSR the greatest tragedy of the 20th Century. He's been in constant aggression against his neighbors. He's also tried to destabilize NATO at every opportunity.  He never recognized the legitimacy of the Ukrainian government. He has said that Ukraine is rightfully a part of Russia based on shared language and history, even though the Ukrainians are a distinct ethnicity from the Russians with their own language, traditions, and history. He has called Ukraine (and other countries that formerly made up the Soviet Union) artificial countries created by the fall of the USSR. He has already annexed Ukrainian territory on the Crimean peninsula. When he made his declaration of intent to invade Ukraine, he barely mentioned NATO. 

If Ukraine had a puppet leader like Alexander Lukashenko of Belarus, Putin might leave them alone. But to what end? Why should Ukraine obey Putin?

Furthermore, if Putin is allowed to take Ukraine unchallenged, it opens the door for a lot more horror. The Baltic republics would likely be next. What about Poland, Romania, Hungary, Slovakia, who are the next countries in line on the borders of Belarus and Ukraine? What about Finland, which sits right near St. Petersburg?

And just as important, would Putin's successes in Europe embolden Beijing to use force to bring Taiwan under its control? 

We are walking a fine line here. Directly attacking Russia would trigger World War III, which would put everybody in danger from Russia's nuclear warheads and 600 million military-eligible Chinese citizens. So we are providing materiel support to Ukraine while cutting off Russia's access to the global economy. The Ukrainians themselves are fighting like hell because they don't want to be ruled by Russia. Ukraine's president is on the front lines along with his people, having said several times that he is willing to die for his country. But we also cannot let Russia and China impose their will on people who don't want to surrender their sovereignty.  If Putin does attack a NATO member, we are obligated to respond in kind.  It is better for if we can help the Ukrainians hold their own long enough for the Russians to decide that Ukraine is not worth the loss of blood and treasure that Russia can't really afford to lose.

Believe me, nobody except Putin wants this war. And if it continues to be a disaster for Russia, Putin is going to wish he'd never started the war.