What I heard/read was that the bad guy had raped someone in the officers family. (Wife/sister/etc) Below is what I found on Deja news, it doesn't mention a rape, but give all the rest of the details.
From Deja News:
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Since 1934, only one legally owned machinegun (of some 100,000+) has ever been used in crime, and that was a murder committed by a law enforcement officer. On September 15th, 1988, a 13-year veteran of the Dayton, OH police department, Patrolman Roger Waller, then 32, used his fully automatic MAC-11
.380 cal. submachinegun to kill a police informant, 52-year-old Lawrence Hileman. Patrolman Waller pleaded guilty in 1990, and he and an accomplice were sentenced to 18 years in prison. The 1986 'ban' on sales of new machineguns does not apply to purchases by law enforcement or government agencies. ---
Thanks to the staff of the Columbus, OH Public Library for the details of the Waller case.