What is a system of a good king? That system covers his own castle, barely. What does it do for the whole country? When you give a single person absolute power, it means everyone under him/her is also looking for similar power, should that fit their personality, and they would rightfully do so. How can you guarantee all of them are "good" as well? A king is a king only as long as he can keep suppressing those who would replace him. Just like a dictator, because that's what a king basically is, only with a fancier title and all kinds of ceremonies and historical justifications. When you assume you have absolute power to control other people's lives, you must also assume your legitimacy only stands as long as you can enforce it. Enforcing it may be quite gruesome.
This is vastly different compared to democracy where the rulers only rule for a limited period of time, under the direct blessing of the majority of the people. You could still technically say the minority, who lost in the voting process, is oppressed, but that's why the constitutions (or other laws) of democracies have elections periodically, so that the losers merely need to wait for the next chance. They don't need to take up arms to risk their lives to change the rulers by violence.
When I say democracy, I mean representative democracy. Direct democracies are extremely rare and, honestly, unless it's an obscenely rich place like Switzerland, where practically everyone is a millionaire, it's probably not worth it. Imagine reading a thousand pages of complicated text every year simply so that you could vote smartly on things that don't necessarily interest you personally, but you still must have an opinion, and it better be a good one because it could have significant consequences.
