As pointed out, because of a history of similar blowback where the West gives weapons to a violent reactionary group that ends up using those same weapons against their former ally. Also, Ukrainian nazis have hinted at the possibility of a "stab in the back" narrative if they don't get what they want. Even himself made threats towards Europe last year.

Curtailing aid to Ukraine will only prolong the war, Mr Zelensky argues. And it would create risks for the West in its own backyard. There is no way of predicting how the millions of Ukrainian refugees in European countries would react to their country being abandoned. Ukrainians have generally “behaved well” and are “very grateful” to those who sheltered them. They will not forget that generosity. But it would not be a “good story” for Europe if it were to “drive these people into a corner”.

Of course none of this is a guarantee that things will happen, but it wouldn't be surprising. If at some point Europe decides that they're no longer willing or able to give them aid, these nazis could pull shit to try and scare them into continuing aid. When you give weapons to nazis, don't be surprised when they come back to terrorize you if they feel abandoned and betrayed.

How are they going to get into the Donbass Republics or Russia?

There may be some sympathizers behind the enemy line but you can’t exactly get into Russia without a strict background check, you know, especially for former combatants and those who are associated with fascism (you know they love their tattoos, which can be instantly identified).

It’s the same reason that most of the 9/11 hijackers (who had participated in Afghanistan or in the Second Chechen War against Russia) were already residing in Europe and America and having free access to America through various visa programs.

Due to history of blowback. Why did the mujahideen turn on the US? Al-Qaeda? Historically America's reactionaries often come back to bite them

It depends on the context of how the war ends realy.

If the war had ended back in April 22 with those negotiations then this would not be a valid prediction at all. But now the west has made promises, ideological statements that they can't easily turn back on politicaly.

Because turning back on Ukraine would mean admitting they're genocidal Nazis all over again, that they shouldn't have supported them etc. All those articles from '14-18 describing Ukrainian far right nazi problem are still there.

So how is the war ending? The most logical conclusion imo is NATO/Poland/Coalition of the "willing" peacekeepers invading western Ukraine pretending the war will continue and that NATO will still defend them while meanwhile the reality is Russia may continue this campaign for another 5-10 years and eventualy get to Odessa and Kiev. At that point Ukraine wont "exist" as a sovereign nation anyway and the Nazis would all have died.

But if this doesn't happen and NATO realy does give up entirely and let the war continue and Russia wins more ground for the next 5 years then the Nazis should strike at the west because their only tactic left will be this blowback bargaining chip.

The Nazis continue to threaten the west because Ukraine is a martyr for western values and this is a political red line for this entire EU political class. If Ukraine turns back on them it means Russia "won" and this is just political suicide. For now I wouldn't expect the Nazis to break this bargaining chip.

So the only other expected outcome is the EU is forced to abandon Ukraine through economic and public pressure and then the Nazis realize there is no bargaining chip advantage anymore.

I’m a bit of a contrarian, I don’t think there will be terrorist attacks in Europe by Ukrainians. I DO think there will be a bunch of Azov Battalion guys and others like them who will declare an openly Neo-Nazi nation within Ukraine, which will either exist as a rogue state or eventually subsume what is left of Ukraine.

In addition to what everyone else has said: the US will order them to. As Europe is forced to deindustrialize and implement even more austerity, there will be a popular reaction - both from the left demanding fewer cuts, more government investment, humane treatment for immigrants and refugees etc. and from the right demanding an end to immigration, even more austerity and tax cuts for the rich, rollback of LGBT and womens' rights, you know, the usual.

The US will heavily fund the latter, while suppressing, intimidating and killing the former just as it did after WW2 in Gladio, the strategy of tension and the years of lead. Ukrainian nazis and mercs have been the go-to for years for all kinds of US ops (here's a photo of an Azov guy eventually wounded on the front (lol rekt) posing in Hong Kong where he was at the """pro-democracy""" protests for... some reason) so once they're even more desperate and fanatical they'll make the perfect terrorist stooges.