A 30-year-old man was allegedly gunned down outside his south Delhi home on Tuesday night by three youngsters who were “fed up of his years of bullying”, police said.Vijay Kumar, deputy commissioner of police (south), said the murdered man, Nishant Sharma, had 17 criminal cases against him, including attempt to murder and under the Arms Act. “He would earlier run betting rackets in north-east Delhi,” said the DCP.But the murder was not directly related to his criminal past, the DCP said after arresting the three suspects in the killing. The officer identified them as Sulender, Sirajudulla and Mohd Umar, all aged between 19 and 20 and residents of Sangam Vihar.“Sharma had been bullying the trio since they were school students four years ago. He would randomly slap them in the neighbourhood or make them kneel on the road in front of everyone,” said the DCP.Sharma was returning from a friend’s wedding reception and was parking his motorcycle outside his home around 9.15 pm on Wednesday when the trio reached there. “Two of them walked to Sharma and shot him in his head, killing him instantly,” said the DCP.