Yellowstone's spinoff Marshals killed off Kelsey Asbille's character Monica, but what was the reason for her onscreen death? The Sunday, March 1, episode of Luke Grimes' new CBS show revealed that Kayce Dutton's wife previously died after she got cancer as a result of toxins dumped onto the Broken Rock reservation. Amid a strong reaction from Yellowstone fans about the offscreen death, Grimes, 42, and Marshals showrunner Spencer Hudnut addressed the narrative decision.