The thing most people, especially Americans, forget or don't like to be reminded of, is an incident North Korea still considers a grave betrayal, and that it was the United States - and not they - that first abrogated the terms of the armistice agreement that effectively ended the Korean War.  Look up what transpired in September 1956 pertaining to paragraph 13(d) of the Armistice Agreement.

I'm not at all suggesting the North Korean government has lived without sin during the succeeding 60 years (especially regarding the kidnapping of Japanese and South Korean nationals) but it's not for no reason or entirely irrational that it views the United States as less than trustworthy.