Ukraine is not in NATO. It moving toward NATO is what Russia is against. You can see NATOs expansion all around Russia as a form of empire building. It does not legitimise Russias actions, but the status quo and ‘natural order’ was abandoned long ago. Behind the scenes Russia and the US/EU have been playing a tug of war over Ukraine for over a decade. And Ukraine was part of the same nation as Russia not all that long ago. So while this doesn’t justify Putins actions, it is more complex than say hitler invading France. Russia was allied with Ukraine when Yanukovych was the democratically elected president. The west, predominantly the US, funded revolutionary groups and helped instigate a revolution that occurred in 2014. Yes Yanukovych was corrupt and beholden to Putin but he was elected by a process that the international community observed as being free and fair. Ukraine had peace and cheap Russian energy, but it was under the wing of Russia. The EU were complicit in undermining this relative order too. See Jack Straws comments who was the foreign secretary around the time - essentially saying the way the west tried to pull these former Soviet states away from Russia and intervene - could have been handled better. And that is a common view. Western interventions completely changed the Russian-west dynamic. It siwed the seeds of this war (and possibly Trump and Brexit interventions by Russia) and restarted a new kind of Cold War. NATO vs Russia are now two forces at war. Literally.
With all that in mind, have a look at Syria. The US, the UK, France etc all put huge sums into arming (predominately jihadist) groups to fight Assad. Did the west think Assad was all that bad at the start of the conflict? No not really. Western educated, a fan of liberal ideals and education, he was seen as a leader we could work with in a very unstable region. Many thought the UK could back him at the outset of the conflict. But politics and alliances formed. It was Iran + Russia backing Assad so the West countered arming Islamic State and the PKK. The upshot? Probably well over half a million dead. Destruction of a nation. And no actual ‘regime change’ (plus what a misnomer that idea can be: look at Libya where we instigated a ‘regime change’ - its now a lawless failed state. A total mess). In Syria fighting continues to this day, with Assad slowly defeating the islamists who are still armed with US etc weapons.
Ukraine has its unique history, and providing some arms so Russia can not walk into the country freely does make sense. But the Stop the War approach is to de-escalate. Not intensify and add arms and fuel to the fire. Stop the War campaigned against the Iraq and Afghan invasion, against Syrian and Libyan involvement and in each of these wars, hindsight has shown them to have been correct.
What do we do now?