The Letters of Louis Farlain and Dr. Hasmors

Louis sends letters to his friend for medical advice with how he's feeling.
It details on about his condition, feeling sick, having pains, and waking up in blood. Louis later finds himself on roof tops and elsewhere as he spirals into madness. While the friend who's a doctor finds the initial letters nothing severe, he later starts looking into these occurrences and linking Louis to a string of murders.