Honestly it's even simpler than all this. A sizeable portion of Donetsk and Luhansk want to secede from Ukraine. Ukraine will not allow them to vote on the matter, and so it has escalated to war. Ukraine is invading two states that have declared themselves sovereign rather than letting the people of the Donbas decide their own fates through voting. Russia has yet to actually invade anywhere and while they should not invade anywhere, it is impossible to say they should shrug their shoulders and do nothing when Ukraine attacks sovereign states between their borders.