Russian, simply can't allow more of the board, specially one part so close, to be dominated by the opponent.
Anyone that think this conflict in terms of "good" or "bad" have been drinking the kool-aid.
My prediction: A repetition of Crimea. The Donbas will be incorporate to the Russian area of influence, but not the rest of Ukraine, some noises will be done by the West about sanctions or whatever and in five years nobody will remember. Hope to be right, because other scenarios are really scary.