If NATO troops had fought on the side of Ukraine, this war would have ended with Russia's defeat long ago. Ukraine receives various types of assistance in defending its sovereignty and territorial integrity from many states, including NATO members, but this assistance, especially weapons and military equipment, is very weak and does not allow Ukraine to achieve a relatively quick victory over the aggressor that attacked it.
But Russia has nuclear weapons and that's why NATO can't help it. NATO is much powerful than Russia and if Russia is about to lose, then what's there to stop it from using Nuclear Weapons? Imagine you have a bomb and 5 guys are beating you to death, won't you use the bomb to kill yourself and your enemies? You are dying anyway, so why should you leave your enemies alive when you have the chance to die with including them?
And now an interesting question - who is Ukraine fighting with now? Because Russia does not have enough strength to seize the relatively small Ukraine, it uses elite units of the North Korean troops on the battlefield against Ukraine, as well as armored vehicles, air defense systems and millions of shells from North Korea. Therefore, in fact, there is a war between Russia and North Korea against Ukraine.
Elite units of North Korea? Elite? They are as Elite as Russian troops.
Your comparison with the guys beating up is a bit unfortunate. The thing is that even if Russia loses this war, it will not mean death for its citizens or the country's elite. It will only mean actual death for the instigators of the war, and there are still various other options for them. For Russia itself, if it does not disintegrate into many independent states, this will be a good chance to pursue a different policy and a chance to eventually join the civilized states. Therefore, sensible people will not grab a nuclear club. Otherwise, the preparation for the use of nuclear weapons will definitely become known to the intelligence services of the world's states and then a preventive non-nuclear strike will be carried out on Russia and in this case physical death for many will become real.
As for the use of North Korean troops in the war against Ukraine, they showed themselves poorly only at the first stage, using the tactics of the Second World War, when the infantry went on the attack in large units. But they learn quickly and in the latest battles have already proven themselves to be a serious opponent. In addition, North Korean soldiers are very physically resilient, shoot accurately with small arms, are fearless and almost never surrender, preferring death. During the fighting, only a few North Korean soldiers were captured by the Ukrainian Armed Forces, and only because they were wounded. There is information that all the relatives of those captured by the Ukrainian Armed Forces in North Korea were shot.