A purge too far

It's tempting to think that Kim Jong-Un had a sneak peek at his Christmas presents and found his uncle's met with his displeasure, the spectacularly public purge of Jang Song Thaek owing to the latter's misjudgment about festive jumpers and Kim's hatred of them.  Now, after it emerges that uncle Jang has been executed, Kim's enmity towards this kind of garment was clearly exceptional.
Jang had been 'purged' twice in the past, his last rehabilitation as Kim Jong-Il made plans for the succession and sought out Jang as a kind of regent.  This was one purge from which Jang could not recover.  It appears that Jang has outlived his usefulness to Kim III - rather than 'a man for all seasons', Jang was 'a traitor for all ages' (though of course Sir Thomas More also suffered the tyrant's wrath).  Maybe Kim was forestalling Jang as Pharaoh Tuthmosis III failed to do with Hatshepsut (the one and only female Pharaoh - Cleopatra never had that title), though Tuthmosis had the excuse of being in a minority.  Some things never change.  After Hatshepshut died from cancer, Tuthmosis conducted a damnatio memoriae, where all mention of her reign that could be found was scrubbed through being too shameful to mention.  Jang is being cropped out or digitally removed from official pictures in North Korea with all the fervour that might be expected in 1984's Ministry of Truth.
Maybe Kim is still rocking from the attempted army coup last year in much the same way that Roman Emperor Commodus was scarred from a botched assassination attempt involving his own sister when he was only 19-years old - he went down as one of the worst, if not the worst, of all the Roman emperors through his depravity, brutality and insanity, yet not so mad as to mistreat the legions.  They were the only ones who mourned him.  Kim Jong-Un is continuing his father's 'military-first' policy, where the army get everything it needs while the populace starve or exist in grinding poverty, the juche ideology of his grandfather, Kim Il-Sung long since quietly discarded (despite North Korea's official website maintaining that "the masters of the revolution and construction are the masses of the people [who] are also the motive force of revolution and construction").
Uncles tend to be bad news for young dynasts.  Hittite King Mursili III tried to curtail the power of his uncle, Hattusili, the victor of Kadesh (the score draw battle with the Egyptians, that was a strategic reverse for the latter under Ramesses II, the Ozymandias of poetry).  This triggered a civil war from which the uncle, now Hattusili III emerged victorious.  In the early Ming period of China, Zhu Yunwen, the Jianwen Emperor, had a similarly peremptory attitude to the power and influence of his uncles, prompting one of them, Zhu Dhi, to sack the capital, Nanjing, with his troops (more than five centuries before the Japanese) and become the Yongle Emperor.  The Jianwen Emperor was never heard from again and his entire reign was voided from the Ming histories.
The USA isn't happy as the downfall of Jang is the herald of instability and it's got enough problems with Pakistan without another nuclear-armed state having a wobble, though technically North Korea's default state is to wobble more erratically than an out-of-kilter gyroscope.  Washington D.C. may have hoped fro Jang to have been a Brezhnevite 'stability of the cadres' figure as the power behind the throne.  Unfortunately, being behind the throne he could not see Kim's plans for the his own evisceration, a blow as swift as when Emperor Tiberius took out Sejanus, the commander of the Praetorian Guard.
Jang isn't the only unfortunate elite to perish at the hands of Kim, the latter favouring the route of Emperor Caracalla (who murdered his wife once in sole power) than Wills and Kate (inviting their ex-s to their wedding) by having a former girlfriend face the firing squad.  There has also been a major reorganisation of the army to weed out those who are not loyal to Kim.
North Korea's rhetoric is so hyperbolic as to give George Galloway a run for his dubiously sourced money.  Jang was "despicable human scum," "worse than a dog," who led a "dissolute, depraved lifestyle."  This Ruritanian backwater would be farcically funny were it not for the millions who died in preventable famines, the expanding network of gulags and the overwhelming abuse of humans rights, to say nothing of liberty, each and every day as a matter of national, capricious edict.