Just my point of view, but right now I think Russia has some good reasons. Assuming that:
1. the Ukrainian rebels are nationalists but not nazis (it would be scaring for me if EU supported a rebellion with such a component)
2. Ianukovich was/is really as corrupt as they say
still, there's something wrong with what the new government wants. As far as I've understood, only the western part of the country is inhabited by ukrainian-speaking people, Crimea talks mostly russian and many territories in the east are also russian-speaking. But the rebels want it all, and they'd rather 'call for help' than giving what would be (imo) right to give. Ukraine has an insane debt with Russia (gas), and I can't see how Russia can be considered as a devil. They want to be free from russian influence and get closer to the EU? They want a state where they (ethnic Ukrainians) can decide for themselves? Good, but what right do they have to get also the lands that aren't really inhabited by them, but by russians? When Czechoslovakia split, two countries were born, now Ukrainians would like in their country something as if the Czechs had taken the whole country from themselves, abolished Slovak language and forced Slovak people to emigrate. Best thing to me would be if the new Ukrainian state gave up territories that are inhabited by russians, Crimea in primis. If you want a long term peace, I don't think there's another option, because if you make russians angry with something less (abolishing russian language and still keeping the territories inhabited by russains) they'll never forgive it (and I wouldn't either). Also Russia is maybe worried about having some sort of natural borders, a potentially hostile Ukraine means the russian plains will be exposed, and this will make them costantly worried and maybe aggressive. They'll consider theirs the lands east of the Dnieper and won't be happy until they take the back, in one way or another. If those lands are mainly russian-speaking, they should change flag. Give them all a referendum, otherwise is it that Ukrainians have right to self-determination, and russian-speaking Ukrainians don't? It really stinks to me.