Does the bomb “legitimize” North Korea, or for that matter does possession of a nuclear weapon “legitimize” any regime? “Legitimacy” does not refer to a universal quality all regimes must have in order to exist, nor does it describe a regime’s behavior at home and abroad. It is simply a concept drawn from international relations syllabuses used to describe how various actors secure and sustain power and prestige.
Or, think about it like this: During the Iran debate, advocates of the deal often argued that the mullahs would never actually use the bomb, or they’d be crazy to use the bomb. Iran, said JCPOA advocates, isn’t crazy. It’s a rational regime.
That line of argument falls away as soon as any power acquires a nuclear weapon. After a state’s nuclear breakout, a central concern for policymakers around the world is that said state may indeed use the bomb. The primary purpose of acquiring a nuclear bomb is to get the world’s attention.