Factual error: In the scene when the police are heading to Clyde's house to arrest him, this tactic is highly unrealistic. In real life, when a person is simply a suspect in an investigation, the police do not all drive to the suspect location with lights and sirens going. This loses the element of surprise. They didn't even know if he was home, anyway.
Factual error: When Clyde is first arrested, he is taken to a prison and put in with the general prison population. People are taken to jail before going to prison. Only after you're convicted do you go to prison. In fact, he wasn't even charged with a crime when taken there.
Factual error: The co-ordinates given by Clyde Shelton: 39° 57' 4"N 75° 10' 22W as the place to find Mr. Reynolds, is the corner of 19th and Sansom Street in Center City (1919 Sansom, to be exact, which is an apartment complex), the downtown area of Philadelphia. The scene actually takes place underneath of the George C. Platt Memorial Bridge (39° 53' 49"N 75° 12' 46W), about 4.3 miles southwest of the coordinates.
Factual error: In the cemetery scene when the SUVs are leaving and the EMP knocks out electrical systems, the next thing you hear is walkie-talkie communucations between the vehicles. Those would've been knocked out, as well.
Factual error: During the lethal-injection scene, an executioner is plainly visible to the witnesses through a broad glass window in the death chamber. He is shown flipping toggle switches in-sequence to release the sodium thiopental, pancuronium bromide and potassium chloride that comprise the lethal drug cocktail. For one thing, in modern Western culture, an executioner is never visible to the witnesses. For another thing, there is no "executioner," per se, flipping toggle switches in the lethal-injection process. The component drugs are loaded into the death machine in advance, and it automatically releases the drugs via a pre-set timer.
Answer: To make sure he was placed in solitary confinement. The warehouse that he owned and operated out of that was next to the prison also had a tunnel connected to every cell in the solitary wing. Clyde needed to be in one of the solitary cells so he could leave the prison whenever he needed to unnoticed, which also served to make it look like he had an accomplice on the outside.
Phaneron ★
When Nick is talking to a spook later in the movie, he is quoted as telling Nick: "That cell-mate that he killed, you think that was random? No. That's a pawn being moved off the board. Anyone who had anything to do with that case, he's gonna be coming after you." Just as all deaths played roles in Clydes game, as the audience we are led to believe this inmate played a role, but were never given any resolution as to what significance it was. Not a big deal in grand scheme of things, but unexplained.
I don't know if you just didn't read the answer thoroughly or if you didn't pay close attention to the movie, but Clyde killing his cellmate was far from being unexplained. He can't leave the prison if he's in a regular cell with the general population, so he kills the cellmate in order to get placed in the solitary wing, because every solitary cell is connected to the tunnel in his warehouse that is next to the prison, which allows him to leave whenever he needs to.
Phaneron ★