Crimea has been in Russian hands more than once. 60% of Crimeans are ethnic Russians. Their Black Sea Fleet is there. I'd say Russia has a legitimate interest.

...the Crimean War, which began in 1853 and involved three years of bloody fighting between Russia and an alliance of the Ottoman Empire, France, Britain and Sardinia. While Russia eventually lost the war and Crimea suffered significant damage, it remained part of Russia.

After the October Revolution ended the Russian Empire in 1917, Crimea briefly found itself a sovereign state. That didn't last long, however: It was quickly dragged into the Russian civil war, where it became a stronghold for the White Army. Following a succession of governments in a few short years, Crimea eventually became the Crimean Autonomous Soviet Socialist Republic in 1921, part of the Soviet Union. It remained like this until 1945, when it became the Crimean Oblast, an administrative region of Russia.

Then, in 1954, something unusual happened: Russia gave it to Ukraine.

(A lot of Russians thought that Khrushchev was smoking crack)

When Ukraine held a referendum on independence in December 1991, 54 percent of Crimean voters favored independence from Russia. It was a majority, but the lowest one found in Ukrain.

http://www.washingtonpost.com/blogs/worldviews/wp/2014/02/27/to-understand-crimea-take-a-look-back-at-its-complicated-history/

By unanimously backing the referendum, the Crimean parliament signaled the intentions of regional leaders as well as Russia to restore the territory to Moscow's control.
The autonomous region has a 60% ethnic Russian population

http://www.cnn.com/2014/03/13/politics/crimea-referendum-explainer/

How many civil wars these days "haven't" ended in a continuing clusterfuck. Look at Syria, three years and over 100,000 dead.
If civil war breaks out in Ukraine, who better to quash it than the Russians? The UN?