The idea that good/innocent/authentic Palestinians are in a Manichean struggle against bad/guilty/inauthentic Israelis is part of a mind-set - a "theory" of sorts - that became dominant on much of the Left after the 1960s. Let’s call it reactionary anti-imperialism. It divides the world, and everything in it, into two opposed “camps”: Imperialism versus Anti-Imperialism. Anyone shooting at Imperialism (the USA, the UK, Israel, "the West", "the Global North", or just "the Man") is now part of the progressive anti-imperialist “resistance” to imperialism. Once in thrall to this ‘theory’, parts of the left redefined themselves as (not very) critical supporters of, or at least apologists for, the reactionary forces doing the shooting, including radical Islamists.