If you understand that Russia is just doing the same thing America usually does it might start to make more sense. Western liberals rabidly defending Ukraine and calling Russia evil is amusing considering they have spent their lives benefiting from the same brutal practices on other countries.

But for a better understanding of this, Russia has for a long time considered Ukraine as effectively their territory or one of their puppet states, much like America does with others. What you are seeing with this invasion is the result of a slow erosion of this notion by western influence. Think of this as a long game aggression similar to how it would be if foreign powers convinced one of the United States' territories to try to secede. That is what people refer to when they speak of NATO aggression. It is all about weakening Russia by removing one of their holds on what used to be their empire. Now they are moving to protect their interests.

You may cry foul at this, but the US would likely find reasons to invade one of it's neighbors (Canada or the United Mexican States) if there was a communist regime change that was propped up by China. You would likely happily argue that the United States has the "right" to per-emptively invade and depose the hostile government to protect its interests. It is strange that you would accusingly ask if Ukraine has the "right" to defend itself, when it could be easily argued that Russia has the "right" to invade. Zelensky is seen as a regime change by Russia and they seek to depose him and bring Ukraine back into their fold.

Considering their prior status, it could be framed that Ukraine is fighting for it's "independence" from Russia, but at the expense of becoming under the thumb of NATO and the rest of Europe. NATO's ambitions for Ukraine are no more honorable than Russia's, and it is the Ukrainian (and Russian) people that will suffer by being caught in the middle of bourgeoisie power struggles between superpowers. The depiction that NATO is merely helping Ukraine defend itself out of the goodness of their hearts is a disingenuous framing of NATO's intentions.

In the end it doesn't matter if a country has a "right" to defend itself, but rather if they are capable of doing so. Ukraine has been on paper an independent country after the collapse of the USSR but functionally seen as still property of Russia, at least notionally. That has weakened to the point where war has broken out. A key take away from understanding the leftist position regarding this war is that we are not very invested in it and don't feel we need to pick a side, but if you really want us to pick a side, most often we will pick Russia. Not because we love Russia, but because we hate NATO.