All systems can decay into tyranny, but what we call fascism is only really possible under capitalism. The closest non-capitalist example was the Soviet Union, which was definitely fucked up but wasn’t really fascist in the traditional sense. Lots of Russian chauvinism? Definitely. Genocide? Absolutely. But no wider ethnic purity project along the lines of Nazi Germany. The distinguishing feature of fascism is its break-neck march towards collapse as more and more people are regarded as untermensch and fewer and fewer people are permitted to be ubermensch. Fascism is essentially murder-suicide as a political ideology (more accurately family of political ideology), and it’s rare if not impossible for a non-capitalist system to produce that.