Clarence Darby

Clarence Darby is the secondary antagonist in the movie Law Abiding Citizen. He is a psychopathic criminal responsible for the murders of Clyde Shelton's wife and daughter, and got away with it by receiving a sentence of 3 years in jail rather than life imprisonment or death, which was the main cause why Clyde became a murderous vigilante in the first place.

History
He first appeared in the beginning of the film, when he and his accomplice Rupert Ames broke into Clyde's home, restrained Clyde, who is then forced to watch as Clarence sadistically murder Clyde's wife and daughter. Though both Clarence and Ames were caught, Clyde is shocked to realize that his lawyer Nick Rice (the hero of the film) has no intention to putting Clarence away for life because the evidence is insufficient (the DNA evidence is inadmissible and Clyde's testimony wouldn't be useful since he blacked out) and that Rice doesn't want to risk lower his high conviction rate. As such, Rice strikes a deal with Darby, who pleads guilty to murder in a very low degree in exchange for testifying against Ames. Due to the plea bargain, he spends only 3 years in jail while Ames is sentenced to be executed, leaving Clyde extremely betrayed. 10 years later, Clyde enacts his revenge on Clarence by tricking him into going from the run from the incoming police following Ames' death (which was caused by Clyde himself). As soon as Clarence reaches by, Clyde reveals himself and paralyzes Clarence with puffer fish venom. Clyde then straps Clarence to a table inside a dark building. There, Clyde enacts his revenge on Darby by cutting him into pieces with a chainsaw, making him die in a slow painful death. When the investigators arrived at the scene, they found him in 25 severed pieces including his decapitated head.

Despite having succeeded in getting his revenge on Darby, Clyde plans to kill more of those involved for Darby's communal sentence. He also sent in a snuff video of himself murdering Darby to Rice's house, which deeply traumatized Rice's daughter, to taunt Rice for making the deal with Darby. In the end, before Clyde's death, Rice admits that he truly does regret making the deal with Darby, stating that he's done making deals with murderers and acknowledging Clyde for teaching him a lesson.

Trivia

 * Clarence is arguably the Bigger Bad of the film because if he hadn't killed Clyde's wife and daughter, the story wouldn't have happened.