Bear with us. The final couple chapters of Red Harvest are a whirlwind of events that can be difficult to follow. This is intentional on Hammett's part. The chaotic buildup towards the ending of the novel reflects the confusion and chaos that the Op feels when he's accused of murdering Dinah. Things are spiraling out of control and the Op is now wanted by the police so one wrong step and he's off to jail. As he tries to stay one step ahead of the cops, the Op manages to piece together the real murderer of Dinah.
When the last of the thugs is killed off, we see that Personville has indeed experienced a "red harvest." All the bloodshed has allowed the city to be cleansed of its "poison," but we're ultimately left feeling slightly unsatisfied. Has the Op really succeeded in performing his job? Will Personville remain untainted or is it only temporary? The Op himself seems to feel cynical about his own success.
At novel's end, he expresses doubt that Personville will remain pure. This note of cynicism is an overall reflection of the sense of disillusionment that was prevalent during this time.