The Autonomous Republic of Crimea had the right to hold local referendums and its separation from Ukraine was legal.
Recurring disinformation. The Ukrainian Constitution reads "The issues of change of territory of Ukraine can only be decided by an all-Ukrainian referendum" (Article 73); "The Autonomous Republic of Crimea is an indivisible part of Ukraine and decides on its policies within the limits determined by the Constitution of Ukraine." (Article 134). http://bit.ly/2eNoUP4
Any decision relating to Ukraine's sovereignty or sovereign choices can only be taken without undue external pressure. This so-called referendum was organised in a matter of weeks by a self-proclaimed Crimean leadership lacking democratic legitimacy and installed by armed Russian military personnel following the seizure of public buildings. The EU does not recognise it https://eeas.europa.eu/headquarters/headquarters-homepage_en/22959/The%20EU%20non-recognition%20policy%20for%20Crimea%20and%20Sevastopol:%20Fact%20Sheet, nor does the United Nations General Assembly http://www.un.org/apps/news/story.asp?NewsID=47443#.Wh7LwGfnCEA.