Thomas Parnell was a serial killer prior to Judgment Day. After being executed, he was obtained by Dr. Serena Kogan and placed in cryogenic storage to become Skynet's ultimate weapon in the future war against the Resistance. With Parnell's ambition growing, he became a threat to Skynet as he wanted to replace it with himself. Hence, Parnell became targeted by both Skynet and Resistance.
Parnell was utilized as a human adaptation by Skynet. This adaptation was intended to bring an individual with human killing capabilities to the battlefield. Eventually, Skynet lost control and Parnell overpowered Skynet and took control over the machines. He then utilized the Skynet assets to slaughter Connor's battalions with massive beagles.