The guard, identified as Neeraj Kumar (28) had opened fire on Satender Singh (35) following an altercation between them in Gole Market area last evening, police said.
Kumar had reached a famous sweet shop at Mandir Marg but had an argument with Singh who worked at the store. Kumar was in a hurry while Singh insisted on him to be in the queue.
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"The altercation soon turned ugly and Kumar, who works with a security agency - took out his gun and shot Singh in his head," said a senior police official.
The victim was rushed to Lady Hardinge Medical College and Hospital from where he was referred to RML hospital, where he died here this afternoon, police said.
Kumar was arrested from the spot and the gun recovered. A case under various sections of IPC has been registered against him, the official said.
Investigators had collected the CCTV footage from the camera installed at the spot in which the incident is said to have been recorded, police said.