Murderers Are Rapists

The alarming tendency of fiction to assume that someone who is physically violent will likely also be sexually violent. This is most common in Fan Fiction, but it also pops up quite a bit in professional works.

There are myriad possible reasons for the existence of this trope. In fan work, it may be a bizarre combination of Draco in Leather Pants and "Not If They Enjoyed It" Rationalization. On the other hand, it could be just the opposite; rape is an easy way to push a character over the Moral Event Horizon in a setting where murder is a frequent occurence, and/or not exclusively performed by the antagonist of the story. Within published fiction, it may be explained by the fact that murderers who double as sex offenders tend to get much more media coverage than your average bread n' butter killer. It probably helps that the writer may want to create a totally unsympathetic villain, and rape is frequently viewed as a Fate Worse Than Death.

Compare Aren't You Going To Ravish Me?, when a character in-story makes this sort of assumption, and Memetic Molester, which is somewhat like the fandom-based variant of this trope, but Played for Laughs and not necessarily applied only to murderers, and Gratuitous Rape, which is a common result. Contrast Rape Is a Special Kind of Evil for when a villain has standards and specifically averts this trope. Compare/Contrast Insane Equals Violent, which by the associative property could lead to this trope.