The accused, Pradeep, had allegedly murdered 24-year-old Marshal Singh and dumped his body near Orissa Sadan in sector 16 of Dwarka.
The victim had been missing since May 24 and his body was found the next day with injuries on the face and neck.
The accused's wife Pooja allegedly had an affair with Marshal's brother Arjun in 2015, said Deputy Commissioner of Police (Southwest) Surender Kumar. She had returned to her husband later but maintained contact with Arjun.
The officer said Pradeep wanted to exact "revenge" from Arjun.
Pradeep was called in for questioning and despite misleading the police initially, he later confessed to killing Arjun's brother.
Pradeep's brother Deepak, who was also part of the crime, is yet to be arrested.
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