Putin said it best: The Ukrainian uprising was a coup. He is intervening to protect Russians. I find that to be legitimate.
But replacing the Prime Minister of Crimea wasn’t a coup? If the government in Kiev is illegitimate, so is the government in Sevastopol.
The “Russians” Putin wants to protect are Ukrainian citizens, unless and until they themselves decide otherwise.
Numerous unarmed protestors where shot and killed. Yanukovich was impeached by just under 75% of their parliament. Yunukovich fled the country. Russian troops should not be in Ukraine.