A bugged-out exterminator went berserk ...fatally plunging a knife into a fellow straphanger's neck after they argued over a seat. Cops rushed to the train and nabbed the alleged killer thanks to a quick-thinking train conductor, who locked the doors. Jerry Sanchez, 37, got on the northbound train at Rockefeller Center about 2 a.m. He confronted Dwight Johnson, demanding he move a bag from the seat where the attacker wanted to sit. Johnson, believed to be homeless, refused and punched Sanchez in the face. Sanchez pulled out a steak knife and stabbed him several times in the neck and face....