Ouroboros

 I might be a bit biased on this one because I'm reading the manga and it is so good. Incomplete manga, translators please help.

But this drama definitely deserves my praise. What a plot. What brilliant acting. What an awesome display of emotions. What a shocking ending, kept me on the edge of my seat.

SPOILERS - The story is about two orphans who witness the murder of their beloved caretaker and are unable to forget or forgive the culprits. The police silence them with threats and the kids grow up in foster homes separately, but they never surrender their desire for vengeance. They renew their promise at their teacher's grave and take different paths to achieve their joint goal, one becomes a cop, and the other joins the mafia. They keep their quest a secret and ask for no aid from outsiders. However, their unusual association raises questions for the people who surround them. Is he a corrupt cop? Is he a confidential informant? Can he be trusted?

The story differs between manga and drama, so enjoy both.

Super Secret Spoilers - Read at your own risk.
The adoption home is an underground market for organ trade. The kids who get adopted vanish without a trace. The teacher was an undercover cop. 

There is a mysterious background to one of the boys, could he be an enemy? Will he change sides, at the last moment? I won't tell you. Watch this to find out.