Rex Heuermann, the meticulous architectural consultant who authorities say murdered three women and buried them on a Long Island beach more than a decade ago, may have been among the last of the dying breed of American serial killers. Even as serial killers came to inhabit a central place in the nation’s imagination — inspiring hit movies, television shows, books, podcasts and more — their actual number was dwindling dramatically. There were once hundreds at large, and a spike in the 1970s and ’