Yeah, Chambliss and Goldberg seem really averse to killing off any of the show's central characters. Of the three they've killed (Nick, Madison, and John), Nick and John's were both because the actors wanted to leave the show.
Yup. He wanted to leave the show as early as season five:
'Fear the Walking Dead' star Garret Dillahunt explains what led to that John Dorie shocker on the show.
ew.com
He's mentioned before that he was really disappointed that he never got to work with Kim Dickens on the show because of how season four's story played out. Between that and the show's less than stellar reception in seasons four and five, I'd imagine he just didn't feel like the show was what he thought it would be when he signed on, though that's just speculation on my part.