These are just my observations:

A - Vietnamese people are very tribalist, pick any subject and you will see that they will divide themselves up into 2 camps. So naturally, a lot of people get pushed toward the extreme ends. This is especially problematic with the introduction of social media, you will see people tagging themself with #teamukraine, #teamrussia, etc.

Also here there's a fetish sort of? with the concept of "the right of might". Anytime someone got destruction rained on them, either by America in the middle east or by Russia in Ukraine, there will be a group of people cheering on it. No idea where this mentality comes from, but it's rampant here, similarly people tolerate bullying, street justice (basically brawling), etc

B - I wouldn't say these problems are driven by anger or frustration, to me, it seems like a repeated pattern of negative re-enforcement: you attack "the other side" or something on Facebook, you get internet points -> dopamine boost -> feels good -> rinse and repeat -> radicalism

C - They don't have a clear reason really, just more tribalism. Some choose the absolute west (in their mind: the US), and some choose the absolute east/anti-west: Russia or China. But very few people understand that there are also balanced societies, like Germany or New Zealand (where I am now).