Yes, they signed and were part of the NPT and then they unilaterally withdrew.[1] But they also are not signatories to the Geneva convention, but we still think they should not torture their own people.
Where are you getting this from? According to Wikipedia, every member of the UN is a party the GC, and North Korea has additionally ratified protocol I, something the USA refuses to do.
https://en.wikipedia.org/wiki/List_of_parties_to_the_Geneva_...