King's Haki is something genetic, this is something most people don't seem to have understood, and that's why this question inherently is incorrect. Genetics are formed before you're even born, so, it's not like you can just claim they shouldn't have something they inherited genetically... might as well just say this person shouldn't have been born.
It's like looking at someone's physical appearance and judging whether they should have a certain quality.
Your genetics are a lottery, not something based on behaviour or appearance.
It would be like looking at someone, and saying this person should never have been tall.. like how do you even determine that? Have to literally be god, or someone capable of manipulating genetics to make such a statement, or in this case, claim someone shouldn't have King's Haki.
Koby might not look like a douche bag, and he may not be arrogant, but that's not what determines King's Haki. It's genes. You either have it or don't, regardless of your appearance or character.
And this idea that people have to have a certain attitude equating into King's Haki is contradictory to the fact we're told you can't train to have King's Haki. You can train or rather, develop your character to be like Kid's or someone who stereotypically is arrogant and ambitious. If all it took was changing attitude to develop King's Haki, then it wouldn't make sense to say it cannot be attained through training, you'd just train/indoctrinate yourself to develop those character traits and then you'd have it.
It's a genetic lottery.