There was illegitimate coup in Ukraine most likely funded by the West. This supported by people in the west of Ukraine, but not those in the East who still supported the old president. Therefore the country became divided with the pro-EU and the pro-Russians. Crimeans wanted to be part of Russia, and therefore had a referendum. Crimea used to be part of Russia until they gave it to Ukraine but now since Ukraine want to cut off Russia, taking Crimea back was done to preserve the status quo. Russia's navy is situated there as well as many ethnic Russians living there.
Now the pro-Russian separatists are fighting the Ukrainian army. Whilst MH17 was a war crime by the pro-Russians, the Ukrainian army itself has also been shelling innocent civilians but ofc western media won't cover that.
Basically Russia is at fault for supporting the pro-Russians and Crimea should of been settled with a more legitimate vote (even with a proper,legit vote Crimea would of voted to join Russia). Ukraine is at fault because it's an illegitimate government who took power through violence and therefore have no right to rule Ukraine.