girl raped in beginning

The assault victim's name is Alicia Rennick and the film makes her Wolkowski's daughter. Wolkowski's recorded voice played during the final showdown with Ken states that she committed suicide and his actions are in revenge.

The child at the film is never identified in the film except by the name Pete, but by association (and the age of Wolkowski's character) the suggestion is that the boy is Alicia's son, and thus Wolkowski's grandson. At the end Wolkowski gives care of the child into his friend from the police who presumably takes Wolkowski away to face judgement for killing Ken Greg and Art.

The book (from which the screenplay was adapted) is more specific in making Alicia the former wife of Wolkowski, that she became pregnant because of the rape, that Pete was the result, and that Ken was the likely father due to Pete's facial characteristics being similar to Ken's.

The book has the rape take place during a college party where she first met Ken Greg and Art. It makes Wolkowski a pseudonym for "George Garner", a local filling station attendant (and Korean war veteran) Alicia's parents forced her to marry once it became clear she was pregnant. Ken Greg and Art get away with the assault and at the behest of her parents Alicia drops out of college in shame. The child was born retarded and Alicia's mind snapped and she is confined to an asylum, eventually killing herself out of guilt. The day after she was buried "Garner" legally changed his name to Wolkowski and undertook a career in law enforcement. He relocated to Ken Greg and Art's home town of Ann Arbor Michigan and over time became the Chief of Police, all in a gambit to take revenge on them himself and use the system he had become an expert in to cover his tracks. (Some of this hinted at in the recording of his voice he plays for Ken during their final confrontation.)

By the time of the actions in the book & film Wolkowski/Garner had got to know the three socially even becoming close enough to be a frequent regular guest in their homes. Ken's wife would often look after Pete when Wolkowski was out of town -- which is where he was at the end of the film, collecting Pete at Ken's home.

By then he had presumably confessed the plot and his killings to his buddies on the Ann Arbor police force, which is why they react as they do ("Well, I guess I'll go tell them") and regard Wolkowski's grandson as "one of their own."