William Goldman is the author of the novels "Marathon Man" and "The Princess Bride", as well as the screenwriter for both film adaptations. Obviously tonight's episode was paying homage to "Marathon Man," and the whole loved one killed by a man with six fingers on his right hand is straight out of "The Princess Bride" vengeance
(My name is Adrian Monk, you killed my wife, prepare to die). Basically, someone behind Monk is a big William Goldman fan (as should we all be), and I wouldn't be surprised to see a Butch Cassidy reference.