Ok so why have NK developed missiles that can deliver chemical and biological agents if they are not developing the agents themselves? Answer they already have.

Why haven't they used them? Answer They like anyone else would have to use them on a small scale, well the biological's at least, because they would be caught in the earths environment and come back and bite the user as well.

Chemical in a war means no rules,total war, and can be answered with the same response NBC, but if we look at where NK is you will see a plethora of reasons the west would want to avoid even passing gas. All that is left is total world war or a long and protrected conventional local war that the west could ill afford.