Police today arrested the 17-year-old son of a constable for the killing of gangster Dharmendra on Monday. The gangster was facing trial for the murder of Constable Roop Singh’s elder son five years ago, and Devendra reportedly shot him to take revenge.On his way to school for his Hindi half-yearly exam yesterday, Devendra, a student of Manaknagar Railway Inter College, spotted Dharmendra, alone at a paan shop, slipped behind him and shot him in the head. Devendra had reportedly acquired a country-made pistol from his village in Jalaun last month for Rs 1,000, and three cartridges.‘‘I saved the money for over three months out of my pocket money. Dharmendra slapped me twice this Sunday too while I was returning from my coaching classes saying he would eliminate me once he was acquitted on December 16 in my brother’s murder. I decided I should act before he did,’’ he said.The boy had reportedly also been facing taunts from classmates, who egged him on to take revenge. The gangster used to extort money from schoolchildren to buy tobacco from nearby paan shops.Police say Roop Singh was unaware of his son’s intentions. ‘‘He gave me a call around 5 pm saying he had avenged his brother’s death. I asked him to come home and them took him to the police station. I’ve lost both my sons now,’’ said Singh.Dharmendra’s enmity with Roop Singh’s family began in 1998, when the constable arrested the gangster in a theft case. On Diwali, in November 1999, Dharmendra got back by bludgeoning Singh’s elder son Manvinder to death. He was arrested and sent to jail, but got out on bail next year. Dharmendra was then charged in an attempt-to-murder case and booked twice under the UP Goonda Act.‘‘Whenever he came out on bail, he started threatening our family. He had even managed to scare away all witnesses in my brother’s murder. This time when he was released in October, he chose me as his target,’’ Devendra said.Yesterday, around 10.30 am, while Devendra was on his way to school on cycle, along with a classmate, he spotted Dharmendra. ‘‘I had been carrying the gun for many days waiting for an opportunity to spot him alone. I stopped my cycle, went behind Dharmendra and shot one bullet into his head. Then me and my classmate both fled. My classmate was not aware of my intentions,’’ he said.Two paanwallahs, present at the scene, told police they had heard a gunshot and then seen two children, in uniform, running away.