Why is North Korea/Kim so hostile?

He's unloved, fat and insane!

If he stopped wasting huge amounts of money on bombs etc . he could feed the starving millions, its only the top brass , who live comfortably.

Because of the U.S. and U.N. sanctions that are making his people suffer and starve.

America killed millions of Koreans in the 1950s their grandparents really America remembers 9/11 with terror and very much regrets it they murdered Iraq for it and are still crying over the twin towers North Korea was flattened carpet bombed to destruction how would you feel if you were North Korean they don't want to be Iraq and Russia and Chine two huge powers next door are not screaming and crying only the USA if mankind has to die why not the Koreans just as decrepit as the USA doing it or Russia one of them will eventually its a certainty

He believes that his taunts will be backed by his big brother, China. He is mistaken.

they want to reunite Korea - their nuke developments are intended to scare of USA, and make them stop helping S.Korea.

Maybe he is upset that he don't think he got as many carrots as America has got.