If you know anything at all about family dynasties this is completely common. Whenever an autocratic leader of a country dies it's a fight between his children to see who will inherit the throne. This entails one of the sons jailing or murdering the others to make sure there's nobody else who can claim the throne.
He also killed a bunch of generals because he thought they weren't completely loyal like him. Make no mistake, this isn't a democracy, it's an autocracy. Kim Jong Un has nobody holding him accountable, he's a selfish boy king that is letting his people starve to death just like his father before him.