"Organizing defence against occupation is illegitimate, but extending one's sovereignity into other countries territory is not?"

Ethnic cleansing is a crime, and it is a far cry from "defence against occupation." That is NOT what the post-Maidan regime was all aboiut. But hey, don't take my word on it, take that of the Nuland-installed PM. Yatsenyuk, who posted the directive for ethnic cleansing of the "subhumans" in the East on the Ukrainian Embassy's own 's website:

https://usa.mfa.gov.ua/en/news/24185-mi-uvichnimo-pamjaty-gerojiv-ochistivshi-nashu-zemlyu-vid-nechistiarsenij-jacenyuk-u-spivchutti-ridnim-i-blizykim-zagiblih-vojiniv-u-lugansyku

The document has been rewritten in the intervening years, and "subhumans" was changed to "inhumans" so as not to be too obvious, but the tone is there.

https://strategic-culture.org/news/2014/06/24/kiev-delivering-on-rhetoric-to-cleanse-sub-humans/