Deek is first encountered in the mission "Spare the Rod, Spoil the Bandit". He is situated along with Williamson on the escarpment, holding a gun on John Marston and Marshal Leigh Johnson's posse. Deek then leads the assault immediately after this encounter. Deek survives, having only minor wounds, and is arrested by Johnson.
Johnson intends to interrogate Deek and exploit him to assist Marston's planned assault on Fort Mercer. Several days after his capture, the Williamson gang kidnaps Bonnie MacFarlane from Hennigan's Stead, and holds her under threat of rape and murder. A Williamson gang member rides to Armadillo to deliver the ultimatum to Johnson, initiating the mission "Hanging Bonnie MacFarlane".
Johnson pragmatically accepts the exchange, carrying a restrained Deek to Tumbleweed alongside Marston, Eli and Jonah. The gang members, awaiting their arrival, ambush them during the exchange and Deek is unceremoniously gunned down by his own gang while being in front of Marston. His death is announced in the Blackwater Ledger.
I'm sure it's been nice for the boys to have a whore to play with. I hear those rancher girls like it in the rear. Maybe she won't wanna go home, she's been fucked so good.