For example, the world in 1914 and 1939 were without a doubt multipolar, and those both resulted in brutal world wars which killed millions.

Do you want to know why? The main contradiction of those world wars right now, was imperialist western multi-polarity, competing to swallow the other over, (with the exception of the USSR and the then-imperial-occupied global south)

The multipolarity we have right now, doesn't contain as much of those contradictions, but, in fact, is more ripe to anti-imperialism, including opposition to comprador capital, capital which not only penetrates, but rather make ravage and dependent a periphery nation to a core empire for its own designs, like with West Africa and France.

This anti comprador stance coincides with not only national bourgeois interest to making their own hegemony, but proletariat, peasant-esque subsistence farmers, and even temporarily-allied petit bourgeois seeking to break their own chains and make their own working class path, the latter who are most beneficial of anti-imperialist efforts. (though national bourgeoisie is definitely a force to vanquish, yet only dissolve when all of the world's capitalists falls with it as well)

That is why we support multipolarity against U.S unipolarity; it challenges, for example, the status quo of dollar domination, with its stranglehold of balance of payments, that force these working class elements in the periphery countries to work to the bone, for not only profit, but give off their trade surpluses to the U.S empire, for U.S prosperity.