Try as I might it's surprisingly difficult to parse the motives of any of the antagonists in "Saimdang: Light's Diary". This is especially awkward since the drama is now at the climax and everyone's getting there comeuppance/unjust victory. Headmistress Choi is jealous. It's dumb but it makes sense. Everyone else? Chi-hyeon is evil for the sake of being evil. Won-soo hates his wife and apparently his children too. King Jungjong is...I don't know, suddenly not appreciative of art any more?
But then really, the answer to any given "why" is pretty simple. Because it's whatever outcome is specifically designed to make Saimdang the most perpetually miserable. I don't doubt that Saimdang's life had a lot of low points. Everyone does. What specifically makes "Saimdang: Light's Diary" so frustrating is that it goes beyond the plausible and simply makes up horrible melodramatic nonsense for the sole purpose of inspiring us to pity Saimdang and everyone unfortunate enough to be in close proximity to her.