I think it is a bit more complicated than that.
Ukraine is a fairly young country and suffers from a split in their populations interest, some prefer Russia and some EU. The west are not enemies of Ukraine, I don't know where you got that from? I don't even think Ukraine were considered an enemy of Russia before all this, sure their were some tensions etc. but that is not the same as referring to them as enemies.
The reason Ukraine can't be part or haven't become part of EU is because they have a lot of issues living up to the rules set by EU, which are in regards to corruption, economics, humanitarian rules etc. that all EU countries have to live up to. Now certain countries joined under the promise of improving these things, which turned out to not really be a good way of doing it. That is why it is a long process to become a member of the EU.
Russia obviously see it as a threat if Ukraine joins the EU and NATO and want to prevent it, so they decided to invade them instead.