We should obviously take this analysis with a grain of salt. First as noted Jang has been "purged" before and then rehabilitated by Kim Jong-il. Second, the reports are only that he has not been seen, there appears to be no conclusive proof yet that he has been purged. Regime members often go for long periods of time without being seen. That said, regarding the execution of his aides one report I received from a Korea hand is that the aides were supposedly executed right in front of him.
Now if Jang has been removed, I think all semblance of adult supervision may be gone. We have to wonder about any remaining influence of his wife and Kim Jong-un's aunt. I wonder what failures Jang is being blamed for? The fact that hostage taking has not achieved significant benefits? Or was he trying to sway Kim to release the hostages? Perhaps he has been advocating following the Kim Il-sung/Kim Jong-il playbook but the US and ROK have not been following it in their responses and since it is not working to Kim Jong-un's satisfaction he had to purge Jang. I think if he has been in fact purged it does not bode well for the Kim Family Regime strategic decision making. Kim Jong-un may be consolidating power but I worry that he has no real idea on how to use that power and we could see a miscalculation.