Many (though far from all) dictators started out with fairly good intentions: Mao, Gaddafi, Mugabe, Castro, etc. all fought against oppressive regimes and were more or less justified in their fight. Turns out that the required skill sets of "freedom fighter" and "effective democratic leader" don't have a whole lot of overlap, and that people with the right skills for both are relatively rare.

Once you've got absolute power fewer and fewer people will call you out on your bullshit and you quickly become more detached from reality – humans are very good are rationalizing our own actions, even when it's wholly irrational, and if there are never any real consequences ... well ...

My point being: Kim Jong Un may genuinely believe he's doing the right thing.