North Korea is the red headed stepchild of east Asia. Its only ally, the PRC, wouldn't support it if it started a war, though it might support it if it was attacked.
Also, although the fact about how 1/4 of their population is military, that's true, but they don't have the largest standing army in the world. That would be the PRC, closely follwed by the USA. North Korea is fourth.
Also, modern military analysts say that if there was a war fought between the North and South, it would be won by the South in a matter of weeks. They have the advantage of 1) much more modern weaponry available and 2) the ability to, you know, feed and pay their troops.
So, apart from the nuclear thing, the South really doesn't have much to fear from the North.