I think it's slightly more illustrative to say that Kim Jong-un had his would-be regent executed. Jang was promoted by Kim Jong-il (Kim Jong-un's father, and predecessor as supreme leader) during the last years he ruled North Korea, and he seems to have had some executive role during Kim Jong-il's illness.
Seen that way, Jang's purge and execution is still a brutal way to eliminate a potential threat but very much in line with centuries of practice in hereditary monarchies all over the world, including those in Europe.
