The double standards are all explained by geopolitical maneuvering. Western powers pulled Kiev into their orbit and threatened to use that influence to harm Russia's economic and military capabilities.
The tl;dr is that Russia needs access to warm water ports. They were happy to lease a port from the Ukraine until rumors began spreading in Kiev that Russia's lease would be ended.[1][2] Because the Russian economy and military couldn't afford to lose access to a naval base with a warm water port they decided to take drastic action.
Another odd footnote about this situation is that Crimea was given as a "symbolic" gift to the Ukraine while the USSR was still strong.[3] It's not like it's some ancient sacred Ukranian land. It's just very valuable for trade and military purposes.
1. https://www.theatlantic.com/international/archive/2015/10/na...
2. https://en.wikipedia.org/wiki/Kharkiv_Pact
3. https://en.wikipedia.org/wiki/1954_transfer_of_Crimea#Compli...
Isn't it evil?
I mean, if you don't believe the results of referendum and subsequent Gallop's poll, than sanctions are intentionally hurting people who are victims of Russian occupation. And if you do believe that overwhelming majority of Crimean people support reunification with Russia, the sanctions punish Crimeans for exercising their right of self-determination and reek of double standards. I don't remember any sanctions on Chechen republic after it separated from Russia, for example.