Alternate possibility: Old Kim was in poor health, weak; power struggle occurred and new leader is decided; Kim Jong Il's death of natural causes -- with a little nudge -- is the last step. This is a not-unknown succession strategy in these kinds of regimes...
![Wink ;-)](./images/smilies/icon_wink.gif)
Further wild but plausible speculation: New leader may or may not be the son; he may be a figurehead for public consumption after he cut a deal to not immediately occupy the grave next to his father.
![evil :evil2:](./images/smilies/evil-2.gif)