The “insanity” label that America attaches to North Korea has a lot of political utility. First, it colors the interpretation of everything North Korea does. The consideration of a rational motivation for undesirable actions can be prevented: the actions are assumed to be crazy. Secondly, it makes the target of blame clear.
A favorite tactic of U.S. propaganda is to label a foreign adversary “crazy” to justify a military attack — as is now happening with North Korea’s Kim Jong Un although his nuclear program really makes logical sense.