Bloody pandemonium erupted in the Borough Hall subway station in Brooklyn Tuesday night when a retired correction officer shot and killed an apparently unarmed man who had confronted him on a No. 4 train moments earlier, police said.

The man who was slain, Gilbert Drogheo, 32, and another man began harassing the 69-year-old retired officer as he boarded a Brooklyn-bound 4 train at the Bowling Green station in Manhattan around 6:30 p.m., according to police sources and a witness.

“They were talking mad trash,” said the witness, 30-year-old Thomas Berry of Crown Heights, who was on his way home from work when he saw the argument. “They must have been drunk.”

The witness said that either Drogheo or his friend told the older man: “Oh, I got you now, my n-----,” as the retired officer tried to walk between the pair to get on the train.

All three men were black, police said, but Berry said one appeared to be Hispanic.