He wants reunification; that does NOT mean he will concede power in the process.
I don't take my information from Western media on this, to be clear. I only take seriously the views of journalists, professors, and the like who have spent extended time in South Korea or are native Koreans themselves.
I say again that Kim wants reunification with the South. He, to my knowledge, does not truly despise South Koreans in general but does despise their political leaders who allow the US to occupy the peninsula. In his eyes, reunification of the peninsula means "liberating" the South from the United States.
Which is great, however, most South Koreans don't want to be reunited with North Korea so where do they stand? What do you think would happen when South Koreans rebel against North Korea, which I'm fairly confident that they would. Again, war would ensue.