They picked the wrong guy to rob.

Off-duty NYPD Detective Benjamin Cintron was headed to work Sunday night when he was confronted by two Brooklyn punks — one simulating a gun and the other wielding a cane — and fatally shot one of them, police said.

“[The officer] thought he was going to be seriously injured or killed. He had no choice,’’ a law-enforcement source told The Post, adding that after the shooting, the cop “was shaken up.’’

Gang member Antwain White, 17, was killed. He and a 15-year-old accomplice had jumped Cintron, a 26-year veteran, in Bushwick at 11:50 p.m., cops said.

The accomplice grabbed the 50-year-old officer’s arm while White smashed him over the head with the cane, authorities said.

Cintron said he was a cop and took out his shield and sidearm. When White hit him again, the detective fired once, cops said.