The people of Crimea however should get to decide where Crimea goes. The only thing Russia can do is either offer to annex or refuse to offer to annex if the Crimean people wish to be part of Russia.
How far should this be taken? Should the majority-Ukrainian sub-districts within Crimea get to decide if their sub-district is part of Ukraine or Russia? How about individual Ukrainian households within Russian-dominated sub-districts?
Managing populations is messy and there is no clear moral principle around geography, but Crimea was a Ukrainian state, under a constitution that required a nation-wide referendum. The question is, at what point are things so bad that you should violate another country's sovereignty?
Russia clearly did more than offering to annex. They actually ran the referendum after sending their military in semi-covertly, ensuring history books will forever question the legitimacy of the vote.