The odd thing is many of the 'spies' seem to be less driven by political ideology and more so by what I could only call being 'true believers' in reunification. Koreans culture is a very xenophobic and homogenous one, and deep down some of them want to be reunited at any cost. It's not exactly nationalism, but it is far more than tribalism.
Korea will never be happy until it is reunited. Some South Koreans can't take the strain of waiting, and seek to help the North and the South become one, no matter the price.