Giles McNamara was the father of Trent McNamara, a victim of The Ghost Killer and Stephanie McNamara, a.k.a. The Ghost Killer. He abused his daughter and locked her in a stable anytime she did anything wrong.
It is revealed in this episode that occurs 20 years after Maya Zinkow's death (a 15 year old girl killed by his daughter) that he raped her, and paid a medical examiner and judge to falsify the evidence and frame a biology school teacher to cover up his crimes. The attention given to another person caused his daughter Stephanie to have a psychotic break and to kill Maya out of jealousy which prompted her to become a serial killer.