Suspect Initially Cleared by Polygraph Now Linked to 1979 Murder of Teen

Polygraph tests aren’t admissible in court for a reason.

> In the 1998 US Supreme Court case United States v. Scheffer, the majority stated that "There is simply no consensus that polygraph evidence is reliable [...] Unlike other expert witnesses who testify about factual matters outside the jurors' knowledge, such as the analysis of fingerprints, ballistics, or DNA found at a crime scene, a polygraph expert can supply the jury only with another opinion."[25] The Supreme Court summarized their findings by stating that the use of polygraph was "little better than could be obtained by the toss of a coin."[25] In 2005, the 11th Circuit Court of Appeals stated that "polygraphy did not enjoy general acceptance from the scientific community".[26]

https://en.wikipedia.org/wiki/Polygraph#Effectiveness

A polygraph is based on measuring normal emotional response.

A psychopath could probably pass a polygraph without any problem since they are not exactly normal.