Legal positivism is one side of the oldest argument in jurisprudence, and it is routinely misunderstood as the position that unjust laws must be obeyed. It is not. It is the claim that the existence of a law and its merit are different questions, and that running them together corrupts both.
Bentham set the terms by separating expository jurisprudence — describing what the law is — from censorial jurisprudence, which criticizes it, and by dismissing natural rights as "nonsense upon stilts". His point was practical: a reformer needs to see the law clearly as it stands before condemning it, and a doctrine that says unjust rules are not really law at all deprives the critic of a target. Austin systematized this into the command theory — law as the general commands of a sovereign habitually obeyed and backed by sanction — which was clear, teachable, and wrong in ways that took a century to state.
Kelsen built the most rigorous version, a pure theory purged of sociology and morality alike, in which validity comes only from a higher norm, ending in a presupposed basic norm at the top. Hart demolished Austin instead: a legal system is not a gunman writ large but a union of primary rules of obligation with secondary rules for recognizing, changing and adjudicating them, and its authority rests on officials accepting a rule of recognition from an internal point of view rather than merely fearing sanctions.
The stakes became concrete after 1945, when German courts had to decide whether Nazi statutes had been law. Hart argued they were law and should have been disobeyed; Radbruch and Fuller argued that rules so unjust forfeit legal character. Hart's answer is the harder and, he thought, the more honest: it forces the citizen to face disobedience as a moral choice rather than dissolve the conflict by definition.