I think if you read up a bit, you'll find the situations different. Russians have been a majority in Crimea for decades (centuries? Idk) and they never slain anyone or invaded that place. That's how it was.
Kosovo has been a part of Serbia since the 6th century, and even during the Turkish invasion of Balkans Kosovo belonged to Turkey as a part of Serbia. In fact, the only period the two were apart was a couple of decades when Serbia got some form of independence from Turkey in the 19th century.
Albanians invaded Kosovo in the 80s, and in the 90s they started killing civilians and burning villages, I believe those paramilitary forces still exist. Basically they would exterminate a Serbian village, Albanians would move in like "Whoops, nobody lives here, the fuck do we know man, we can just move in here yep!" and nobody took the blame because the paramilitary was never recognized by Albanian government. Serbian response to that was sending their own army to exterminate Albanians and whatever, which was utterly wrong but yeah, a nation just came in and took a piece of land that was never theirs before. Hell, even Israel belonged to the Jews SOME time ago.