The exterminator accused of savagely stabbing a subway rider to death in an unprovoked attack was a ticking time bomb, his devastated aunt said yesterday.

“Oh, no, what did he do this time? Did he finally kill someone?” said Lisa Rivera, 45, when The Post asked her about Gerardo Sanchez, 37. “I’ve been afraid of that for a long time.”

Sanchez, of The Bronx, was arraigned Sunday on a charge of second-degree murder in the slaying of Dwight Johnson, 36, aboard a northbound D train at 2 a.m. Saturday as about 30 riders watched in horror.

“He’s been in and out of trouble most of his life,” Rivera said of Sanchez. “He was arrested a time or two when he was younger for starting fights.”