In Saw, at the end of the movie we learn that Zep was forced to hold the Gordons captive to obtain an antidote for the slow-acting poison in his body. However, this scene confuses me, because Zep hasn't done anything wrong to Jigsaw. He was just an attendant to him.
Why did Jigsaw poison Zep?
Why did Jigsaw choose Zep to hold the Gordons captive?