Jack the Ripper had a frontal lobe injury.

 My suspect had a frontal lobe injury. He was shot in the forehead. What's interesting here is the mention of witzelsucht; Jack the Ripper's letters took a mocking tone.

This lobe is often cited as the part of the brain responsible for the ability to decide between good and bad choices, as well as recognize the consequences of different actions. Following a frontal lobe injury, an individual’s abilities to make good choices and recognize consequences are often impaired.
Euphoria and disinhibition are described. The euphoria is not that of a manic condition, having an empty quality to it. The disinhibition can lead to marked abnormalities of behavior, sometimes associated with outbursts of irritability and aggression. So-called witzelsucht has been described, in which patients show an inappropriate facetiousness and a tendency to pun.

 Witzelsucht (from the German witzeln, meaning to joke or wisecrack, and sucht, meaning addiction or yearning) is a set of rare neurological symptoms characterized by a tendency to make puns, or tell inappropriate jokes or pointless stories in socially inappropriate situations. A less common symptom is hypersexuality, the tendency to make sexual comments at inappropriate times or situations. Patients do not understand that their behavior is abnormal, therefore are nonresponsive to others' reactions. This disorder is most commonly seen in patients with frontal lobe damage, particularly right frontal lobe tumors or trauma.

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