Clyde Shelton: I'm gonna pull the whole thing down. I'm gonna bring the whole fuckin' diseased, corrupt temple down on your head. It's gonna be biblical.
Nick Rice: You think your wife and daughter would feel good about you killing in their name?
Clyde Shelton: My wife and daughter can't feel anything. They're dead.
Nick Rice: You end this!
Clyde Shelton: I'm just gettin' warmed up.
Nick Rice: Are you seriously going to answer that phone after all the shit you've given me for using mine?
Judge Laura Burch: Well, that's one of the benefits of being a judge. I can do anything I want to.
Clyde Shelton: You're the one who makes deals with murders yea? Well I've come to make mine. Release me.
Nick Rice: Or what?
Clyde Shelton: Or I kill everyone.
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 ★