NEW DELHI: An 18-year-old youth was stabbed to death by four men after he allegedly confronted them for harassing his friend's younger sister in west Delhi's Kirti Nagar on Sunday night.Family members said that the deceased, Manoj, was known to the men who attacked him since childhood and were once closest friends, but Manoj opposed them when they started harassing his friend's sister. The four accused - Sonu, Raman, Rahul and Munna - had attacked Manoj in the past as well."We registered a case and started a manhunt for the youths soon after we received information about the incident. Several teams have been formed and the men will be nabbed at the earliest," deputy commissioner of police (west) Vijay Kumar said.Manoj and the accused often had fights and arguments over the harassment. Manoj had even registered a police complaint against the accused youths, but the matter was resolved amicably later. The four accused had also filed a case of kidnapping against Manoj.On Sunday, Manoj's friend asked him to accompany him and have a word with the youths who were harassing his sister for the past year at Kirti Nagar's Chuda Mandi.The duo reached the area and found them drunk. When Manoj confronted them, he was stabbed in the throat with a knife, after which the youths fled."All of them were drunk and started bashing them up as they were about to leave. They stabbed Manoj in the neck from behind when he was leaving," said the friend's sister, adding that her brother informed the cops.Police said that they got to know about the incident around 10 pm, soon after which a team was rushed to the spot, but within five minutes, a huge crowd gathered at Manoj's house and created a "nuisance"."A separate case under sections of rioting, obstructing the functioning of policemen at duty and causing hurt was also registered later in the evening," DCP Kumar said."Manoj always treated the girl as his sister and was very protective of her," said Suman, a relative.Police said that Manoj and his friend were thrashed by the four youths several times in the past. "Both the groups had registered FIRs against each other in the past and were apprehended for the same," the DCP said.A case under Sections 323 (punishment for voluntarily causing hurt) and 341 (punishment for wrongful restraint) of the IPC were registered against them a few days ago, but even after repeated police intervention, the two groups clashed. Police said that there were times when they compromised and withdrew their complaints.