Ending questions Spoilier
Why did he confess if he was innocent? My guess is just because everyone doubted him from the beginning and he knew that they'd all feel like idiots when they found the real killer.
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Hearst's "confession" could have been an act of contrition....atonement, perhaps even self-immolation. Not for murder, which he didn't do, but rather for the decadent, false (even his hairpiece), corrupt lifestyle that had been turned inside-out and upside down...right in his face with the questioning by his former friend, the police captain and Hearst's rival, Lt. "Opie."
This is reflected in the last scene in the plaza, when Chantall and Hearst sit apart, amongst the revelry. For the first time in their marriage, all pretense and false front has been stripped away...leaving very little.
Morgan Freeman's character stares from the balcony at the couple. What is he thinking ? Is he stunned by his own complicity in this incident ? Is he amazed at the transformation of Hearst before his eyes ? Does he wonder where the "justice" lies in all of this mess ?
The answers are left up to you.
CmdrCody