It really comes down to the country's leadership. All of the other points in other comments here are certainly valid, but he feels that NK's leaders are more likely to be willing to push the button and the US is in range.

What I can't say is if this belief in NK's leaders is based on actual intelligence or some sort or just his opinion. Given their willingness to take pot-shots at SK, I know I'm concerned about this.

Because the regime has always been crazy, even as it passes through different rulers. The best case scenario of North Korea is that the world is faced with a humanitarian crises on a shocking scale that will cost tens of billions of dollars to correct. The expected lower limit of the results of foreign action working to take down the North Korean regime would be the deaths of hundreds of thousands or millions of South Korean civilians (in a matter of hours) due to shelling from thousands of DPRK artillery positions. The possible upper limit of badness would be the North Koreans hitting Los Angeles, San Francisco, Seattle, Taipei, Tokyo, and Seoul with nuclear and/or chemical weapons on ICBMs, IRBMs, aircraft, or smuggled onto commercial cargo ships.

Not really an ideal frame of reference

The Axis of Evil included North Korea along with those two countries.