A store security guard fatally shot a gunman who’d shot and killed another man Thursday afternoon near a busy intersection in the South Shore neighborhood, according to Chicago police.

       The shooting began as an argument between the 25-year-old gunman and a 22-year-old man around 2:50 p.m. in the 7100 block of South Jeffery Boulevard, police said in a media notification.

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       As the argument escalated, the older man pulled out a handgun and shot the 22-year-old in the torso, police said. A security guard, a part-time police officer for Cook County health care, according to authorities, saw the shooting and opened fire on the gunman.