Over the course of a decade, two prolific serial killers terrorized the women of Baton Rouge, Louisiana, raping and murdering, dismembering and cannibalizing. Neither of them ever met, but each of them knew what the other was doing and, perversely, it appears, they were trying to outdo each other in terms of their depravity and sexual deviance. The converging stories of Sean Vincent Gillis and Derrick Todd Lee are told in a gripping true crime mini-series, “Butchers of the Bayou.”