Was Kosovar people taking Kosovo illegal? Who determines illegality? You acknowledge that almost all the inhabitants were ethnic Armenians.

Your calling it "illegal" is you 100% just complying with the "might makes right" principle. Current military powers (like the USA & Russia) make right in accordance with their will; the USA used its might to make the Kosovars and the South Sudanese right in their struggle for democratic secession. India used its might to make Bangladesh right in its secession from Pakistan in the 1970s. No mighty power, however, helped out the Armenians of Artsakh/Nagorno-Karabakh, and so in your eyes, they were not right. That is the only difference between Kosovo/South Sudan/Bangladesh on one hand, and Artsakh/Nagorno-Karabakh on the other. Screw democracy and self-determination. Might makes right. (I'm being sarcastic.)