Well seeing as the ukrainian government is pro-russian anyway, I don't see Russia doing any harm by deploying troops there to stop the riots.
Never mind that it's actually illegal to deploy troops in another country without the consent of the UNSC. With the Budapest Memorandum in 1993 Russia signed an agreement to respect the borders of Ukraine. Sending troops into Ukraine without the consent of either the UNSC, nor the internationally recognized interim government of Ukraine constitutes a gross violation of that treaty.
Also, there was
significant voting fraud involved when the Crimean parliament decided to denounce the Kyiv government and declare a local criminal its prime minister. Several of the representatives who allegedly voted for separation according to protocols, weren't present at the meeting. On the other hand, Russian soldiers were there, watching the show with weapons in hand. And the referendum they are supposed to have on Sunday is kind of fishy too. The alternatives are "Join Russia now" or "Go back to the 1992 constitution of Crimea and its relation to Ukraine". However, the 1992 constitution of Crimea, later nullified by the Kyiv government (a decision anchored in the 1993 Budapest memorandum) declared Crimea independent. Staying part of Ukraine is thus not even an option in the referendum. It's "Join Russia now", or "Declare independence and then join Russia". Looks like it'll break apart no matter what. Perhaps that's the reason why Russian troops are already building border posts and burying land mines along the roads between Crimea and mainland Ukraine.
Russia sending troops without insignia also qualifies as a minor war crime. Technically speaking, the masked, and suspiciously well-equipped, troops in Crimea are unlawful combattants and not protected by the Geneva convention. As for protecting the people of Crimea, they didn't do a very thorough job, seeing as houses of Crimean Tatars (generally considered to be pro-Ukrainian) were vandalized by painting large X-es over their doors. The Crimean Tatars would have been the ethnic majority on the peninsula, hadn't they all been deported under Stalin's rule in a single night in 1954 (those who live there now are descendants of those, after they were allowed to return to Crimea in 1988). The deportion to Uzbekistan killed almost half the Tatars, and started when Soviet soldiers broke through their doors in the middle of the night. How did the soldiers know which doors to break through? Why, the doors where Crimean Tatars lived were marked by large X-es.
As for the overthrowing of Yanukovych, well, that's a complicated matter. It was technically illegal under the current Ukrainian constitution, which Yanukovych pushed through in 2010. It extended the powers of the president, and made it harder to impeach him, among other things. However, according to the deal signed by both the government and the rebels, signed on February 21, the 2010 constitution of Ukraine was officially to be scrapped and the 2004 "Orange Revolution" constitution immediately reinstated. The decision passed through the legally elected parliament of Ukraine, but lacked Yanukovych's own signature to be officially approved.
However, things got too "hot" for Yanukovych, who fled the country before he could sign the constitution swap. He was impeached shortly thereafter, through absolutely legal means according to the 2004 constitution. However, technically this constitution wasn't in effect at the time of impeachment, but the only thing preventing it from being in effect was that it lacked the approval of the impeached president. A complicated case, indeed.
Keep in mind that it was democratically elected officials who voted Yanukovych down. The composition of the Parliament didn't change during the Maidan protests, these representatives were all voted in during the last election. It's just that a lot of Yanukovych's supporters were absent from the voting, thus making the opposition the majority when the vote was cast. Still, enough MPs remained to make the vote legal according to Ukrainian law.
Last, Russia seems very eager to stand up for Crimea's right to follow the will of its people, its right to declare independence if the population so desires. A few hundred kilometres from Crimea lies Ingushetia and Chechnya, two Russian republics which have fought bloody wars for independence for decades, and where less than 2 % of the population are ethnically Russian. Looks like the Russian support for independence is restricted to regions who want freedom to
join Russia, not to leave it. Want to go the route of "Crimea was historically a part of Russia" instead? Well, that's certainly not the case for East Prussia. If recent historical connections are so important, why don't Russia give back Kaliningrad to Germany?
F-ing hypocrisy, that's why.