Would the USA leaving NATO end Russia's war on Ukraine?

NATO is basically a U.S. promise to defend Europe. You are suggesting that NATO be dissolved. Sure, Russia would like that, but Europe would not.

Only if France and the UK did as well if the goal is to remove the chance of nuclear powers shooting at each other. The goal of NATO, as I recall, was to defend Europe and give Germany such a warm fuzzy that they didn't feel they have to rearm. The last 4 times they did so were, umm, messy.

And perhaps the biggest fallout from Ukraine (aside from the undesired fallout itself) was Germany announcing rearmament and its neighbours thinking it cool.

No. Russia isn't actually threatened in any meaningful sense because they have a gigantic military and over 6,000 nuclear weapons. 0 Countries represent a threat to them militarily.

If the U.S. left NATO, Russia would actually invade more countries as far as we currently understand Putin's motivations via public media channels. He wants to recreate most or some elements of the Soviet Union which include Ukraine, but also other countries such as Estonia, Latvia, Lithuania, Moldova, Georgia, Poland, and others. Had the United States not been part of NATO, it's incredibly likely that what remained of NATO would have taken no action while Russia forcefully seized Ukraine.

The best solution here is that Russia is a peaceful democracy like other countries in Europe and instead of wielding hard power, it could wield soft power and become a substantial global economic partner in many ways, improving the lives of its citizens and enriching itself in the process. The problem is that those in power are literally mafia-style criminals who through one form or another obtained wealth and power after pillaging the remnants of the Soviet Union.

Nobody in NATO has any desire for a war with Russia for any reason. Everyone wants Russia to respect human rights and trade in a prosperous arrangement. Again, the people in power in Russia don't want that. That's their active choice.

It might halt their current invasion because Putin could claim it as victory from the complete mess he's gotten Russia into. But NATO potentially expanding to Ukraine wasn't the reason Putin attacked Ukraine.