1. Hinterkaifeck

2. Villisca Axe Murders

3. The Axeman of New Orleans

4. The Moonlight Murders

5. Mad Butcher of Kingsbury Run

6. Jack the Ripper

7. The Zodiac Killer

Maybe don't read these before bed.
On the night of March 31, 1922, the six residents of Hinterkaifeck, a small farmstead in Germany, were murdered with a mattock (a tool similar to a pickaxe). The victims were Andreas and Cäzilia Gruber, their widowed daughter Viktoria, her children Cäzilia and Joseph, the maid, Maria Baumgartner. It was rumored that Andreas and Viktoria had an incestuous relationship, and that Joseph was their son. The Gruber's earlier maid left the family six months earlier, having claimed the home was haunted. And days before the murder, Andreas told neighbors he'd discovered a strange set of footprints in the snow leading from the forest to the home, but not back. He'd also heard footsteps in the attic, and a set of keys went missing. Based on the crime scene found days after the murders, investigators believed the family was led one by one into the barn to be killed, before the murderer killed the maid and young Joseph in the house. More than 100 suspects were ultimately questioned, but none were ever convicted of the crimes.
On the night of June 9, 1912, the well-liked Moore family (including four children aged 11 and under) and two other children, friends of the Moore's daughter Katherine, were bludgeoned to death by an axe in their home. The bodies were found the next morning, their heads covered in bed cloths. Among the suspects of the murders were a drifter named Andy Sawyer, a visiting priest accused of pedophilia named Reverend George Kelly, and two men separately suspected of being serial axe murderers. A string of axe murders across the country prior to and following the Villisca murders closely resembled each other (mirrors in the house were covered; gloves were worn by the killer; a basin in which the murderer washed himself off found in the kitchen), but were never officially connected. Both suspected serial killers, William Mansfield and Henry Lee Moore, went on to murder their families two years and several months after the Villisca case, respectively, but neither was definitively tied to Villisca, and the case remains unsolved.
Between 1918 and 1919, an axe murderer killed eight people in New Orleans and surrounding communities, with more victims suspected. The majority of the victims were Italian-American, and most were killed with axes that belonged to them. On March 13, 1919, someone claiming to be The Axeman published a chilling letter in the newspaper which, in part, read: "I am not a human being, but a spirit and a demon from the hottest hell. I am what you Orleanians and your foolish police call the Axeman. When I see fit, I shall come and claim other victims. I alone know whom they shall be. I shall leave no clue except my bloody axe, besmeared with blood and brains of he whom I have sent below to keep me company." The Axeman's identity was never discovered, and the murders stopped as suddenly as they started.
Between February and May of 1946, and unknown serial killer called the "Phantom Killer" shot 10 people (five of whom died) in and around Texarkana. An urban legend grew around the case, stating that victims were attacked by the full moon; however, this claim was unsubstantiated. Residents of the area were thrown into a panic, and many chose to nail down their windows and/or purchase firearms. Early victims Mary Jeanne Larey and Jimmy Hollis, who survived the attack, described the Phantom Killer as a very tall man wearing a white sack over his head, with holes cut out for his eyes and mouth. Nearly 400 suspects were arrested over the course of investigations, and numerous people made (presumably false) confessions, but none were convicted.
Also known as the Cleveland Torso Murderer, the Mad Butcher of Kingsbury Run was a serial killer active in Cleveland in the 1930s. The official victim count is 12, but investigators believe the true number is likely higher. The Mad Butcher was so named for beheading and dismembering his victims (beware the Google image search for this one; the above picture is a plaster cast of one victim's head, but much more gruesome pictures exist), and sometimes castrating his male victims. Many of the victims were not found for many months (or even upwards of a year) after their murders; some were never identified as their heads were never found. Some suspected there was more than one "Mad Butcher," but the case remains unsolved.
Jack the Ripper is the name given to a serial killer active in poor London communities in 1888. There are 5 "canonical" murders with strong ties to Jack the Ripper, though 11 murders overall have been variously tied to the same figure. The killer's MO was to slash his victims' (mostly female sex workers) throats and viciously mutilate the rest of their bodies, occasionally removing some of the victims' organs. The precision with which organ removal was done led some to believe Jack the Ripper had medical expertise. Several letters purporting to be from Jack himself were publicized around the time of the murders, including the one that appointed the name "Jack the Ripper," though this one was later said to be a hoax. Another letter, known as the "From Hell letter," in which the author claimed to have fried and eaten half of a victim's kidney, was thought to be authentic. Delivered with the letter was a small piece of human kidney in a box. There are over 100 people named as suspects, but none were convicted.
"Zodiac" was the self-appointed name of a serial killer active in northern California from the late 1960s into the early 1970s. The Zodiac claimed, in various letters sent to newspapers, to have killed 37 people; 7 victims are known, 4 of whom comprised young couples, and 2 of whom survived. Most of the Zodiac's victims were shot in or near their cars. Among the correspondence sent by the Zodiac killer to newspapers were four cryptograms, only one of which has been definitively decoded. It reads, in part: "I LIKE KILLING PEOPLE BECAUSE IT IS SO MUCH FUN IT IS MORE FUN THAN KILLING WILD GAME IN THE FORREST MAN IS THE MOST DANGEROU[S] ANAMAL OF ALL" (sic). Various people in the last several years have claimed to be, or claimed to be related to, the Zodiac Killer. The case remains open, but unsolved.