After he wins the fight he has lost the perpetrators so he takes to the rooftops to catch up/find them.
Scene 10: Detective catches up
He sees them and slides down a gutter/ladder/pipe to get off the roof.
Scene 11: Alley
The two people are fighting in an alley. The non-murderer seems to be winning.
Scene 12: The decision
The detective turns the corner and sees them fighting, the non-murderer has a knife on the murderer, the murderer is holding his arm trying to stop the knife. From the detective's point of view, it looks like the innocent one is the murderer.
The detective pulls out his gun but doesn't know which one is the murderer. As the civilian is about to win against the murderer they both stop as they notice the detective and his gun.
We hear a gunshot and the screen goes black. The audience does not know whether the detective shot the right man.