It's a lot cheaper to supply weapons to Ukraine to fight Russia than to fight Russia directly. And clearing out the old stockpiles is good for the companies that are making new stockpiles. If this war turns out to be a big waste of time and materiel and lives for Russia, maybe they don't do it again in the near future; maybe other countries with plans for border expansion see the results and don't do it in the near future either. (OTOH, maybe they see this and figure out air superiority and combined arms operations are important and work on that capability so they can get from invasion to occupation quickly)
If you want to invade a soverign state (especially one that you've signed treaties about and with), you've got to either do it with some sort of aquiescence of the UN security council, a coalation of several countries with sufficient military, or at the very least, do it quickly and efficiently. If you move in a large convoy with no international support and no fuel supplies, you're going to find the locals well supplied by your global rivals. If your first week or two of operations makes occupation look inevitable in the near term, that illicits a different response; but neither the USSR nor the USA/coalition were able to reform Afghanistan into what they wanted. Asymetric insurgency with outside support is almost certainly guaranteed.
You have everything upside down in your head. Helping Ukraine does not destabilize Europe. Letting Putin win does, because destabilizing and dividing Europe is his main goal. He does not stop in Ukraine because winning there will simply confirm his concept of winning wars.
Another wrong aspect is that you imagine that Russia is like UK, a democratic country. It is not, Putin has turned it into a Fascist regime. If you asked what is wrong in Nazi Germany's attack against Poland, then you would be asking the right question - you don't realize why it is wrong because you are not asking the right question.
Neither democracy nor autocracy are ideologies. Whether Russia is democratic or not is independent of whether it is in the interest of the west to be friends with Russia.
Communism is dead. "Autocracy" is not a ideology in opposition to "democracy." Rather nations and regions have interests. The interests of the West, especially the powerhouse economies of the EU are better served by friendship with Russia than by pouring money into some Ukranian nationalist black hole.
Ukrainian People's Republic https://en.wikipedia.org/wiki/Ukrainian_People%27s_Republic
In fact, current Ukraine is successor of this state.
I do not know what in your mind are European interests but to me it is wider acceptance of European ideas - building prosperous democratic societies.
European leaders believed that deeper integration of Russia with Europe will help it to move exactly in this direction. Instead it milked Europe to build an army to destroy it.
Russia did not attack Ukraine because its resources, it attacked it because it was moving too close to Europe and became too Unrussia.
With some exceptions, Europe is a union of democratic nation states, and in fact some of its members splitting up into multiple nation states would only strengthen it. Ukraine is perfectly fine for Europe and joining the union through the accession process will make it much better country.