why would a leftist who hates fascism go join the Russian army?
Reminder that you are in a bubble. Do not shape your perception of reality based on what you see on this website. It is populated by posts from a relatively small amount of people who lean in the same political direction on many issues.
Russia has been calling the Ukrainians Nazis for years. Putin pointed at AZOV and painted all of the nation in the same broad brush. De-nazification is how Putin labeled his actions. Fascism and Nazism are words that have lost all meaning at this point. The muddling and devaluation of those words is leading our world on a path to authoritarianism.
When you view the matter from the lens adulterated by different propaganda the answer becomes obvious. To him the question was probably "How could a leftist not flee his capitalist homeland and father who is complicit to the oppression of the people's cause to join the successor of the USSR in their fight to restore the former Union's territory and drive the 'nazis' out of it."
No problem. To add on being able to identify the way others view matters and the rational they employed to justify their actions allows us to identify logical reasons from their perspective for them to change their opinions.
On point A, I suspect he self-radicalized at first and then entered his own bubble where he further leaned more into radicalization.
On point B, I believe it does. It doesn't matter what the negative label is; viewing individuals as the caricatures that float in our minds when we hear words like "tankie" or other words that serve only, for us, to brand individuals as part of a incorrect collective encourages us to make these same un-empathic mistakes.
We shouldn't like many of these people we brand as things like "Tankie"; but we should understand why they feel the ways they do.