From 1919 to 1924, he committed at least 27 and possibly over 50 murders, many of them with an accomplice, Hans Grans. His victims were young vagrants who hung around railway station, whom Haarmann would lure back to his apartment and then kill by biting through their throats in a kind of sexual frenzy. Haarmann and Grans would then peddle meat from the bodies of their victims as black market pork, turning numerous unwitting Germans, starving in the bleak post-World War I depression crippling their country, into cannibals. When one of Haarmann's customers did get suspicious of one of his purchases and notified police, he was told he should be grateful to get such fine meat in such hard times.
Haarmann and Grans were eventually apprehended when numerous skeletal remains, which they had thoughtlessly tossed out of a window into the river below their lodgings, washed up. Haarmann made a spectacle of himself at his trial, shouting verbal abuse at witnesses and complaining that he was only being prosecuted for 27 murders when he could remember over fifty.
Haarmann was beheaded, though Grans received only a twelve-year sentence. What became of Grans after his release is not known.
Haarmann became known as "The Butcher of Hanover." A film titled The Tenderness of the Wolves was released in Germany in 1973 dramatizing Haarmann's crimes. It starred Kurt Raab as the killer and featured Rainer Werner Fassbinder in a minor role.