You are maybe confused with neo-Nazi Azov battalion and co. also funded and trained by the USA, which have committed documented war crimes against civilians.
That reason? To push Russian forces out of their country.
If Russia weren't terrorizing the region, Azov would not exist. Russia pays to astroturf the claim that Azov is some thing that appeared out of nowhere when it's a group that formed directly to keep Russia and its violent militias out.
In fact, if the US can interfere in a country that's thousands miles away by funding extremist military groups, I find it way more legitimate for Russia to fund Russian separatists in the Donbas region, which has historically had strong ties with Russia and whose population would rather stay there than join the "Western" side. It's also interesting how everybody always talks about self-determination, but only in the case where people want to "self-determine" themselves in the right direction. I would be interested in knowing why it doesn't apply in this case.
To conclude: the Ukrainian army along with this these neo-Nazi groups (that have been effectively integrated into the army) has destroyed several hospitals, schools and other civilian infrastructure and killed more than 3000 civilians since 2014 in the Donbas. Ignoring this situation and blaming everything on Russia is one of the reason that we are in this situation right now, and surely one very myopic way of assessing what's going on.
Azov and the Taliban are bad, yeah. But they appeared because of foreign threats. None of this would happen if Russia didn't repeatedly murder people in neighboring countries then backpedal and retroactively justify their invasion by saying radical defense forces are defending themselves from Russia.
Don't want your country to be attacked by extremists? Don't invade and push people to radical groups. Your paycheck signer deserves the blame for Azov 100%.