SARAH WANGARI’S home in Mathare, a slum in Nairobi, is hardly spacious. The corrugated tin walls are covered with sheets. In one room a tiny bed fills half the space; posters of Kenyan reggae musicians and Haile Selassie, Ethiopia’s former emperor, provide a dash of colour. The next room has been closed for three months. It belonged to Ms Wangari’s 19-year-old son, Alex, who was shot ten times at close range by police. “They said he was a gangster,” she says, gesturing at the room. “But a gangster would not live here.”

Alex died at around 10.30pm on November 18th. His mother and her neighbours say he was walking home with a friend when he was killed, having spent the day carrying water for not much pay. Ms Wangari came across her son’s body as she went to buy milk. “I saw my child lying down, and six police told me to go home, so I ran,” she says. The next morning she returned. At the sight of blood on the ground, she cried. To add exploitation to her misery, at the city mortuary, she was told to pay 20,000 shillings (around $200) to release her son’s body.

She says she knows who killed her son: a tall, middle-aged officer known as “Rashid”, who was among the group around her son’s body. “I saw him with my own eyes. He is a killer. He has killed so many people, so many people’s children.”

Walk around Mathare, a district of densely packed tin shacks and crude brick apartment blocks, and Rashid’s name comes up again and again. Mothers tell their children not to misbehave, lest Rashid come for them. People tell fabulous tales: that he is the son of a major politician, a devil worshipper or even a member of the Illuminati. No one is sure where he comes from, or where he lives. But they all agree on one thing: he is a plain-clothes police officer who kills people.

Omar, a 24-year-old who runs a small kiosk where young men gather to chew qat, describes his modus operandi: “He has a list. If you are on it, you don’t last a night.” Fred, with a bundle of green sticking out of his cheek, is more concise: “He is your worst nightmare.”

How much of this is true and how much slum mythology is hard to gauge. What is certain is that in Mathare, which is home to perhaps 250,000 people, killings by police are so common that they are considered normal. The Mathare Social Justice Centre, a community organisation, says it has counted 803 reports of police killings in the slum between 2013 and 2015. It has documented dozens of these, interviewing witnesses and victims’ families. This is not difficult work, says Kennedy Chindi, who goes by JJ, the organisation’s co-ordinator: often the killings happen in broad daylight. He says that a week rarely goes by without at least one. Last spring Kenyans on social media shared a mobile-phone video of a man, who Mr Chindi says is Rashid, shooting dead a suspect who is lying on the ground and has apparently surrendered, in Eastleigh, close to Mathare.

When the police eliminate someone, they do not file a report. Sometimes the dead man’s relatives are even asked to pay for the bullets. Elizabeth Ndinda, a slight woman whose son, Simon, was killed in early January by a different police officer, says that neighbours helped her raise the money to release her son’s body. This is evidence that he was innocent, she argues: “If the boy was a thief or a thug, people would have agreed with the killing.”

Many do agree with the killings. Thieves are fiercely resented, for their victims often go hungry. Rashid is “doing a good job,” says Moses, a 32-year-old clothes seller. “If someone snatches my phone, they deserve it. The young people should work, not eat from others’ sweat.” Ordinary police are corrupt, and gangsters, if arrested, can buy their way out of jail. Near Ms Ndinda’s home is a strip of shacks where people brew changaa, illicit alcohol, to sell across the city. Most cops come in only to extort bribes from the brewers. Rashid, by contrast, is seen as brutal but mysterious and incorruptible, a sort of slum Judge Dredd. Many in Mathare insist that he rarely kills innocent people, or that he warns his suspects to shape up before shooting them.

Killer cops are not new in Kenya. Patrick Shaw, a white settler who stayed on after independence in 1963, used to shoot suspects, then go to their funerals and haul in grieving associates for questioning. When he died in 1988, President Daniel arap Moi praised his “untiring service to law and order”. As recently as 2013 the Daily Nation, Kenya’s leading newspaper, called him a “one-man judicial system”.

There is little evidence that such lawless policing works, however. Violent crime still thrives in Nairobi. In poor districts, robbery and extortion are routine. In a survey by the International Police Science Association, an American NGO, Kenya’s police were ranked the third-worst in Africa, after those in Congo and Nigeria.

Wangui Kimari, an academic, says that Kenyan police kill with impunity because the poor are seen as criminal. The rich can kick up a fuss when they are mistreated by police. But even they are not wholly immune. In October Bunty Shah, the son of one of Kenya’s leading industrialists, was killed at his family home when police raided it, claiming to have intelligence about a weapons stash. In a country where the police are judge, jury and executioner, no one can feel safe.