I've been reading about Hwaseong Fortress in Suwon and how it was built as a result of filial piety King Jeongjo had towards his father, Prince Sado. However, from the memoirs of Sado's wife, Lady Hyegyeong, it seems obvious that Sado was a terrible person (he raped, murdered, and tortured people on the regular), as well as had severe mental health issues that made him afraid/anxious (to the point of harming others) about things as simple as getting dressed in the morning.
My question is, why would Jeongjo want to build something to honor such a monster? I'm assuming it has to do something to do with Korea's deep sense of Confucianism at the time. I also guess Jeongjo probably had a bit of trauma from his father's execution when he was eleven (Sado was forced into a rice chest by King Yeongjo, Sado's father and Jeongjo's grandfather, until he died seven days later). If anyone has a fuller answer, please let me know.
It is precisely because of your question that many Korean historians have made efforts to reexamine the traditional narrative of Prince Sado.
The basic foundation of any historical methodology is source criticism - why are certain narratives propagated? What biases exist? And so forth.
To accept Prince Sado as just another insane royalty is to accept the two major sources - the Joseon Dynasty Annals (written by court scholars who likely belonged to the dominant Noron faction of Neo Confucian bureaucrats) and the Memoirs of Lady Hyegyeog (also perhaps under the sway of her family who were major figures of Noron) - uncritically. Rival historians and factional intrigues of the times continue to influence our image of various historical characters today, if you need another example check out Byzantine history from the Secret History to the Chronographia and the Alexiad. It is because if these issues we run into in our examining of sources that we need to reexamine and revise our current understanding of history as necessary.
The consensus on Sado has shifted quite a bit over the last decade or so, no doubt renewed by interest in his life in the public sphere. But it is because of Yeongjo's naming of Sado (思悼 世子 thinking with grief) and Jeongjo's subsequent restoration of honors to his father that makes historians believe this is worth reexamining.