It could have been curtains for her. Accused Gilgo Beach killer Rex Heuermann nearly offed a member of his high school drama club in a backstage incident, a former classmate told The Post. In 1980, Berner High School in Massapequa on Long Island put on a production of “Arsenic and Old Lace,” the macabre Joseph Kesselring play about a deranged pair of sisters who lure, poison and bury a dozen old men in their cellar.