Sulk is the young black man who works alongside Astor. When Mrs. McIntyre learns that Mr. Guizac has arranged for Sulk to marry Guizac's sixteen-year-old cousin (who is dying in a Polish detention camp), Mrs. McIntyre, flips out. Interracial marriage is not something she considers acceptable, even if it could possibly save the girl's life.
Sulk, along with Mrs. McIntyre and Mr. Shortley, contributes to the murder of Mr. Guizac by deliberately failing to warn Mr. Guizac that the tractor brake has slipped. Why would he do this? Well, in Section 1, we learn that Mr. Guizac caught Sulk stealing a turkey, manhandled him, and dragged him before Mrs. McIntyre. Sulk, however, wasn't punished for this transgression. Mrs. McIntyre apparently prefers him stealing food to paying him enough to buy his own. The fact that Sulk has to steal in order to eat might well be humiliating him, and something he doesn't want Mr. Guizac to know about. If he also feels that Mr. Guizac cheated him – taking his money to pay for his fiancé's trip, but using it for something else (any number of people could have put this idea into his head, and for all we know it's true) – then he might have wanted revenge on Mr. Guizac.
Like Mr. Shortley, Sulk leaves the farm after Mr. Guizac's murder. How do you think the incident might have affected him?