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Unnecessary postulate. The Left eats its own because no one can be sufficiently “pure” in ideology and the Left cannot compromise. From the Jacobins onwards this is an immutable law.
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Possibly, but not necessary. For similar reasons to 1., Leftist ideological purity is an uncontrolled (and perhaps uncontrollable) weapon. There must always be a fresh target on which to unleash the wrath of the morally superior. Once the genie is out of the bottle, Leftist fanatics must draw blood. It matters less who supplies the blood, so long as they can be defined as enemies of the cause.