I don't remember all the details, and some of details I remember may be false. Here is the plot:
German (?) movie, 1970-2000, Novel adaptation. Murder thriller. Respectable old man leaves a restaurant (or opera?) and go kill another man for no reason. Then throw the gun to the river (nobody finds it). On the trial he said no one word, and got his life sentence (or just very long sentence). In exactly 1 year he hires a journalist (or advocate?) to prove, that he's innocent. The journalist investigates the whole case, and by the end of the movie proves in court, that the man didn't commit a murder. But personally he understood, that the man did, and even have found a reason: a victim has done something horrible to the daughter of the man (raped, or killed, I don't remember).