Well, China and Russia *do* consider the DPRK a threat: just not as much to them as it is to the U.S. and its allies. They don't really want nuclear weapons to spread freely throughout the world any more than we do. The DPRK basically served to counter American, ROK (South Korea), and Japanese interests in Asia. It's basically like China/Russia trained a guard dog to be really mean. They just want to keep people off of their property, but don't want it to break out of the yard and maul anyone. Now, not only are they afraid the DPRK will break the leash, but it may also turn around and bite *them*.
The DPRK has enough artillery pointed at Seoul (where about half of South Koreans live) to wipe it off the face of the earth. They also could hit Japan, Hawaii, Alaska, or possibly California with a nuke if they managed to make a working missile. The DPRK is *definitely* a threat to the U.S. and its allies, and does not seem to use any rational method of decision making as far as anyone can tell.