Naresh Dhankhad, 45, rampaged through a two-kilometre stretch between Agra Chowk and Camp Colony from 2am to 4am, taking advantage of the low visibility due to dense fog.
He killed his first victim on the second floor of a private hospital, before hitting the streets to kill five others and injure one.
A resident of Macchgar near Ballabhgarh, Dhankhad also attacked policemen when they tried to stop him.
He was working as a sub-divisional officer with the health department.
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Dhankhad apparently has suffered brain hemorrhage and was admitted to New Delhi's Safdarjung Hospital.
"He was brought to the emergency services at our hospital and has been shifted to Ward B surgery i.e. neurosurgery. He will be operated upon by the neurosurgery team," the hospital's spokesperson said.
Dhankhad's first victim was a woman, named Anjum, who was an attendant of a person admitted in the ICU of the hospital. She was sleeping on the second floor where the accused managed to gain entry, Palwal Superintendent of Police Sulochana Gajraj said.
He then went to Agra Road intersection where he killed three more people. Then a little farther, he killed two others by repeatedly hitting them on the head.
Gajraj said as soon as the police received information, they started sending people, who were either sleeping or working outside, indoors.
She said checkpoints were set up in the town, which is 80 km from Delhi. "When we zeroed-in on him, he was searching for another target and almost had his seventh victim as he had injured him," she said.
Dhankar tried to resist arrest and even assaulted the policemen before being overpowered, she said.
The SP said that Dhankar appeared to be mentally ill. However, doctors would be able to throw more light on this, she added.
"He appeared to be mentally ill and looked highly aggressive. Preliminary investigations have revealed that he had strained relations with his wife and two children," she said.
Deputy Superintendent of Police Abhimanyu Lohan said prima facie it seemed that he carried out the killings without a reason.
A high alert was sounded in the city, the police spokesperson said.
The spokesperson said Dhankhad got married 10 years ago and separated from his wife four years later. Dhankad was the youngest of five brothers.
His brother, Chandrapal, claimed that Dhankhad was mentally ill and was being treated by a doctor in Muradnagar, the spokesperson said.
A doctor at the hospital at Faridabad where the accused was brought by the police told reporters that he was unconscious at the time he was wheeled in.
The neighbours at the place, where the former Army officer lived, told reporters that he did not like to interact much and preferred to be alone.