The Pledge by Friedrich Durrenmatt

The brilliant Inspector Matthäi is haunted by the case of a young girl brutally murdered in a Swiss mountain forest. Even after a man is arrested and confesses to the crime, Matthäi is convinced the real killer is still out there. He makes a promise to the girl's mother: he will stop at nothing to bring the murderer to justice.

But Matthäi's pledge becomes an obsession, and soon he finds he is descending into the darkness himself...