Complex sentence: If the utility monster existed, it would justify the mistreatment and perhaps annihilation of everyone else, according to the mandates of utilitarianism, because, for the utility monster, the pleasure they receive outweighs the suffering they may cause.
Split into simpler sentences: If the utility monster existed, it would justify the mistreatment and perhaps annihilation of everyone else, according to the doctrine of utilitarianism. For the utility monster, the pleasure they receive outweighs the suffering they may cause.