If the narrator of a story is a prisoner serving time for murder and rape, what kind of language would you expect and not expect?
I think that much of this is going to depend on the characterization of your murderer/ rapist. This is going to determine how the prisoner's language and verbal expression will form. It should be noted that it might be capitulating to stereotype if the prisoner speaks in a manner that offends readers. Certainly, history points to this. Mass murderers like Ted Bundy were described and polite and charming. Gary Gilmore was highly insightful and skilled at chatting, so much so that his story was made into a film based off of the Norman Mailer book. It makes sense that their language patterns were not offensive. They had to lure their victims through some level of charm. I think that if the narrator were following their forms, the language might not be entirely offensive and disgusting, but would actually be verbal patterns that would enable a sense of the attractive to emerge, in order to allow their desires and needs to be met.
However, in another light, if the narrator is struggling to control these urges and embraces rehabilitation, the language might be different. It is important to keep in mind that the language chosen for the character is going to be entirely dependent on what the plot of the narrative will be and how the characterization of the narrator is going to fit into this. Constructing language and verbal patterns of recognition is most effective when it reflects plot and characterization.