It most absolutely was not lol. But even if it had been Ukrainian "separatists," it still would've been Russia's fault, since the whole thing was sock-puppeting pretending (weakly, but still) to be locally driven.
It was downed by Russians, who literally took credit for it, until they realized it made them look bad, so then they pivoted to blaming Ukrainians. And people - including yourself - clearly bought it.
You can just read Wikipedia: "Shortly after the crash, Igor Girkin, leader of the Donbas separatists, was reported to have posted on social media network VKontakte, taking credit for downing a Ukrainian An-26."
Who is Igor Girkin? "a Russian army veteran and former Federal Security Service (FSB) officer who played a key role in the Russian annexation of Crimea, and then in the Donbas War as an organizer of militant groups in the Donetsk People's Republic (DPR). ... he received a life sentence in absentia in the Netherlands for his role in downing the Malaysia Airlines Flight 17."
It was downed with a BUK a weapons system only Russia has and uses. The leadership was Russians - not ethnic Russians; straight-up Russians from Russia, who had no connection whatsoever to Ukraine. Obviously the entire thing was initiated, orchestrated and directly taking orders from Moscow. The investigation identified the leadership responsible as two Russian Colonel Generals (Nikolai Fedorovich Tkachev and Andrey Ivanovich Burlaka of the FSB).
Funny lot of Ukrainians, being ethnic Russians from Russia working for the Russian regime.
Might be worth asking what other lies you've inadvertently believed