Jason Nelson

Jason Nelson is the main antagonist of the episode of The Pact of Criminal Minds.

Background
Very little is revealed about Jason Nelson's life, other than being a serial killer who killed raping girls including Kelly Taylor as her disappearance was caused by Mark Logan, one of three friends who were with him. And initially Logan raped her and after murdering Taylor, and unbeknownst to her friends, Nelson discarded her body. While Logan accepted her disappearance, Taylor was sentenced to ten years in prison, since his body was never found at the time of the crimes.

The Pact
After the two vigilantes kill Mark Logan and Paul Montgomery, who are the two men involved in Kelly's murder. They then confront Nelson, who pretends to have remorse and leads to Kelly's body in exchange for his life. Then when the three go to the Russel's SUV funeral. Nelson then reveals his true nature, and tells that he had expected someone to come to him since Logan and Montgomery died, and asked the neighbor to write down the plague of any neighbors who appear strange and call the police. Then Nelson makes Russell kill a passing driver by shooting him, and Nelson taunts her by saying that she killed a driver for no apparent reason. And Nelson tries to persuade Russell to join him, but Russell threatens him by saying that she will kill him if he does not lead to Taylor's body. After that Beckett asks Nelson where Taylor's remains are, which he replies dismembered her body and scattered it in a few different places. Enraged Beckett attacks Nelson brutally and kills him.

Personality
Although much has been said about Nelson, he has been shown that besides being a rapist and a pedophile, he was a cruel psychopath who killed a three girls without any remorse, he also succesfully manipulated Ellen Russell and gave persuaded her to joins him before his death at the hands of Darlene Beckett.