Dr. Sanjeev Dhawan, 51, who was working in the hospital for the past 20 years, was shot at around 2.45 a.m. at his Rajinder Nagar residence in central Delhi. He died on way to hospital, police said.
Accused Dharmender Kumar Tandon, 30, a former army gunman working as a security guard at suburban Gurgaon's Palm String Hotel, was arrested from Karol Bagh Metro station, police said.
"A gun that was used by the accused and an axe have been recovered from his possession," said Additional Commissioner of Police Devesh Chandra Srivastava.
Police said Tandon, who hails from Bilaspur in Chhattisgarh, loved the doctor's maid Sangeeta, who had been working at the victim's house for the last 25 days.
"Tandon wanted Sangeeta to leave the work and live with him but she refused. A few days ago, he came to the doctor's house and threatened him to let her go with him. He also threatened Sangeeta," he said.
"Last night around 10 p.m., Tandon called up the doctor and threatened him," said Srivastava.
"He consumed alcohol and took an autorickshaw from Gurgaon to the doctor's house. First he went to the third floor of the house, where the doctor's tax assistant Tarun Kalra, 40, lives with his family. He manhandled Kalra," he said.
"The doctor came out of his first-floor room after hearing the commotion and was shot at by the accused, who fled soon after," the officer added.
Sangeeta, who hails from Kanpur in Uttar Pradesh, told police that she met Tandon at the New Delhi Railway Station one-and-half years ago and lived with him at a house in south Delhi's Sarita Vihar.
"After a few months, Tandon allegedly had a fight with the landlord and injured him critically. In that case, he was sent to Tihar," said the police officer.
The officer said Sangeeta later broke her relations with Tandon and took up a maid's job at the doctor's house.
Tandon joined the army in 2000 and left the job in 2010 due to a medical problem. He told police that he was married and had three children.
Dhawan is survived by his wife Jyoti Dhawan and two sons - Sarthak, a law student, and Samarth, a medical student.
Condoling the death of doctor Dhawan, D.S. Rana, chairman, board of management, Sir Ganga Ram Hospital, said: "He was a very social, cheerful and helpful person."