A 67-year-old retired havildar, reportedly mentally unstable, allegedly bludgeoned his 70-year-old sister-in-law, to death with an axe at her house in Mohali around 2 am on Friday.The deceased has been identified as Karnail Kaur, a resident of Mamupur village near Gharuan.The accused, Mohkam Singh, has been arrested and the axe allegedly used in the crime has been recovered.A complaint was filed by the victim’s son, Hartej Singh, at the Kharar Sadar police station.In his complaint, Hartej told the police that Mohkam lived in a shed near their house and reared buffaloes near his fields in the village.“On October 29, at around 2 am, my uncle came to our door with an axe. Out of fear, we did not open the door. My mother, who was sleeping in a separate room, opened the door of her room. Mohkam hit her on the head, fracturing her skull,” said Hartej.He further told the police that he called Mohkam’s son, Manjot Singh, and informed him about the incident. Manjot and his brother, Gurkirat Singh, came to the spot and caught their father. They also threw away the axe. With the help of his cousins, Hartej said, they tied Mohkam up, locked him in a room and informed the police.Mohkam had retired from the police force as a havildar around seven years ago.SI Balwinder Singh said no revenge motive or family dispute had been established yet. A murder case had been registered and the accused arrested. Further investigation is under way.