I like to think the main reason for support for the support right now is the right and obvious one. If Russia had successfully executed a blitzkrieg I’m not so sure Ukraine would get the same support to take it back.
Bonus reasons, however, vary throughout Europe but the main concern for EU, foremost an economic union, is always above all economic stability, short term and long term. Everything is treated as if it’s an economic problem. Climate, wars, pandemics (how can we get out of this with minimal economic damage). Without a stable economy, history will repeat itself, nationally and internationally. Economic power is to EU what nukes are to Russia. Mutual assured destruction of the economy. It’s not as much about US/NATO/EU vs Russia military threat here as it is about US vs EU vs China’s economy.
Another factor that plays a role is Turkey. EU market cannot (nor does it want to) include both Ukraine and Turkey. The decisions it should be Ukraine and it should not be Turkey have already been made. Good thing for them Erdogan will win, EU would have been obligated to restart membership talks with the other guy while EU membership for Ukraine is 10-20 years away. One of the biggest opponents of Turkey joining was Merkel / is Germany. Their support has little to do with atoning for ww2, it’s all about protecting economic power.
The perceived existential threat was always, since the early 80s anyway, more specifically a nuclear war between superpowers US and Russia than specifically Russian nukes. A not uncommon idea here was that the US made Russia seem stronger and more dangerous than they were. What good is being a superpower and spending so much on military if you have no adversary to measure up to. Rocky IV, where a swedish giant represents Russia and an Italian dwarf the USA…
Obviously there’s a change in mentality, already started before this war (though Germany…), the trolling, hybrid and digital warfware, and with Putin going mad. But regardless of the outcome of this war, the nuclear threat will likely remain, or at least still perceived to be real. The support for Ukraine is despite increasing the risk, not in hopes they’ll be less of a threat when it comes to nukes. Sure we now doubt Russia’s nuclear capabilities too but even if a fraction works we’re fk-ed. How fast EU will be to reestablish relations with Russia (after Putin) will underline the above. The believe in the power of economic relations is near religious, it will bring democracy to Russia like it did in China (I’m being sarcastic of course).