When Paris was stabbed, Harry Kim made a deal with Zio: if Zio gave him supplies to clean Paris' wound, Kim would take Zio with him when he disabled a chute so they could escape. After disabling the force field that protected the chute, Zio and Kim discovered that the hatch opened into space, and that the prison which they thought was an underground facility was really a prison in a huge space station.
Zio was the only inmate who could control the mental effects of the clamp that was placed on each prisoner. Kim believed that Zio was insane. Zio wanted Kim to kill Paris because he was a burden and drain on their supplies. He told Kim that in order to survive, he would have to become a killer. (VOY: "The Chute")