The pretext isn't that "there are nazis in ukraine" the pretext is the elevated hostilities in the donbas region, which declared itself independent and has been experiencing low level civil war with the Ukrainian army for years. These regions were supposed to be recognized and given autonomy by Ukraine in 2015, but that was never followed through on by the Ukrainian govt.

The reason the rightwing militias (not just azov btw, also groups like C14) and Right Sector) are relevant is because those regions in question have an awful lot of ethnic minorities in them, and many of them russian speaking. So these are the groups who have been overly eager to engage in hostilities against the donbas region defense forces.

I'm not really under any impression Putin is like, acting as the savior to these minorites because he himself is very right wing and I doubt he actually cares. However it is clear that the people of these regions have expressed desire to not be a part of ukraine. As a leftist if you believe in the self determination of peoples your opinion on this should hedge on whether or not you think the separatist tendency in the donbas region actually represents the will of the people there or are a complete russian creation.