If you mean the character, he left after a serial killer, Frank, killed a friend of his. The actor, Mandy Patinkin, left because of differences with producer Ed Allen Bernero.
The actor Mandy Patinkin left after he was absent from a table read, he didn't come back. The week before he told a reporter he was uncomfortable with some scenes, and he thought it was sick how viewers loved the show so much.
