It's really unreal that in this day and age, this kind of purge would occur. This is something that happens in Imperial China, not in the 21st century!
I think the fact that Kim felt the need to go to these lengths show that there is significant factional support for Jang. I doubt that killing Jang's blood relatives will quiet the dissent. I think we're about to see some political upheavals in NK...
Furthermore, this incident proves that Kim has no intention whatsoever of modernizing North Korea. Can you imagine a modernized nation tolerating that kind of blood purge?
I recently read some articles from a Chinese source about Chinese businesses planning to invest in North Korea. Their line of thinking was that they had the opportunity to make as much money from NK reforms as others did from Chinese reforms.
I think Kim's actions have proved that that line of thinking is just a pipe dream. As long as the Kim dynasty rules, there will not be any kind of reforms in North Korea.