Which raises the issue.
Was the extermination of Jang Song-thaek et al justified because of a real threat or was it merely paranoia?
If the unrest is real and significant then there may be political justification for the very old school purge
If there are concerns that the “king” is too young and erratic, taking out the older, powerful guy before he takes you out is reasonable to prevent your own purge.
You must be hallucinating
You know this dude has wives and children executed too?
He needs to go down
Besides, Kim wanted Jang’s money. The stories coming out of NK are that Kim's own slush fund was running low and he wanted to have control over all slush funds. Jang balked.
To make matters worse, well-fed Jang’ private army (securities for hard currency earning fishery enterprise) kicked the asses of regular soldiers whose size is three times of the former in military's attempt to take over the enterprise. Time was on Jang’s side. Jang’s power was gaining. I doubt that he plotted outright takeover when he was arrested, but Kim saw that it is only a matter of time before he is relegated to a mere figurehead unless he does something.
Another problem is that Jang took away lucrative hard currency earning enterprises from powerful military interests. He also took away things from other outfits. The aforementioned “Internal Affairs Troop” was taken away from Organization Guidance Department. He made a number of enemies. He capped Ri Yong-ho, a top military man known as the best field commander in NK at the time, and took over Ri’s cash cows.
So it is probably natural that Kim Jong-eun and these disgruntled elites teamed up to take Jang down.