Sasohan Geojinmal

Older brother Yu-Chan excels in his studies. Younger brother Yu-Jae shows no interest in school. Their mother Ji-Won is relieved when Yu-Jae starts opening up to his tutor, Mi-Ra. But one day, Ji-Won receives a shocking letter from Mi-Ra: "I write to you regarding my murder of your son."
(Source: Manta, edited)