Killer Impression - S11.E27

In 1987, the death of Crystal Purcell was considered an accident. Then in 2001, Barbara Purcell called police to suggest that her estranged husband Willard had killed Crystal, who was her husband at the time of her death. Before that investigation could begin, Barbara was found dead in much the same manner as Crystal. Was this an unfortunate coincidence or the M.O. of a serial killer?