Why does kim jung un keep threatening the world when everyone including himself knows he will be blown to pieces if he even dared to attack?

Because Clinton gave him nuclear technology and materials to be quiet plus a large sum of cash.

The U.S., said Clinton, would finance the fuel shipments and the reactors if the consortium fails to do so. The total value of the U.S. pledge is esti- mated conservatively at more than $4 billion. In addition to leading the international energy assistance consortium, Washington has pledged to ease its long-standing trade embargo and move toward first-ever diplomatic relations with the North.

THEY HAVE GREAT REASON TO BELIEVE THE USA WILL COLLAPSE AGAINST RED LINES IN THE SAND.

The guy is insane.

To maintain internal power

He is a thug and a bully in his own country. Looks like he is dumb enough to believe it works on everyone.

He figures China and Russia have his back against America, and the South Koreans don't want a war or its consequences either, even if they were on the winning side. And, he uses threats of war in his local politics to keep his own people beat down and scared, and thus easier to control.

bluffing

Kim thinks Trump is weak like Obama.

I think you saw that when Saddam Hussein agreed to get rid of his wmds, he was invaded anyhow and he was hung. So, as a result of that, North Korea is not going to negotiate over nukes, and is taking a hard line. We can thank Bush and the Republicans for the problems we're having today with North Korea. We're not going to attack North Korea, because they will send their nukes towards Japan and South Korea, strong American allies.

he knows Trump is unqualified to be president and has the mind of a child

China doesn't let US do that.