Confession of Pain (2006)

When veteran detective Hei finds out that his father-in-law, the billionaire benefactor Kim, is gruesomely murdered in his palatial mansion, he enlists the assistance of his former partner turned private detective, Bong. On the surface, the murder smacks of a vendetta that has taken a lifetime to fulfill. But no sooner has Bong agreed to crack the case with Hei than he realizes nothing is what it appears to be. Undoubtedly, they are after a monster in a perfect crime: every detail was meticulously orchestrated, every motive conveniently justifiable, and every culprit and potential witness mysteriously eliminated. But Bong has his own demon to fight over the suicide of his pregnant girlfriend. As he digs deeper and deeper into the case, he starts grappling with the suspicion that the man they hunt is someone very close to them, someone on the verge of a total breakdown.