Christos - the Crimea was originally a gift to a fellow Soviet State, (they were part of Soviet Russia during the time of the Soviets), before it had been Russian for a long time. Due to its important strategic position and due to what the present Russia sees as encroachment of the West (American missiles from Germany installed now in Poland for example) - the pro-West sentiments of the Ukrainian leaders and again the strategic advantage Crimea would give the West, plus a still existing pro-Russian population in that part of the Ukraine led the Russians to support the separatists in Ukraine's East, who are largely ethnic Russians.
The Prime Minister at the time of the downing of the plane was an intellectual with pro-Western attitudes (Arseniy Yatsenyuk) and the feeling prevailed that the Ukraine should try and join the EU. In 2013 there were demonstrations and unrest in Kiev's Maidan Square which became known as the 'Euromaidan'. The then prime minister had to abdicate and Yatsenyuk took his place. These developments further increased Russia's fear of territorial encroachment of the West - closer to Mother Russia's heartland.
(We, in Australia have little understanding of those feelings, calling an entire island continent our home; yet still we need to align ourselves with powers we would need or could find useful some time in the future.)
