In Part 2, Chapter 4, Alexandra describes Frank to Carl. The way she hones in on his jealousy foreshadows the violent end of Frank and Marie's marriage:
He’s jealous about everything, his farm and his horses and his pretty wife. Everybody likes her, just the same as when she was little. Sometimes I go up to the Catholic church with Emil, and it’s funny to see Marie standing there laughing and shaking hands with people, looking so excited and gay, with Frank sulking behind her as if he could eat everybody alive.
Frank seems to have a great deal of pent-up energy waiting to explode. Not only does he feel jealous, but he also looks as though he is barely holding himself back from "eat[ing] everybody alive" when he watches Marie happily shake hands with everyone at the church. The narrative principle of "Chekhov's gun" holds that details like this will have some significance later on in the story. Just as a gun is rarely introduced into a story without later going off, readers can be fairly certain that Frank's jealousy is going to manifest in some kind of violent act.
There are some more specific clues in this passage, too, about the way Frank's jealousy will be triggered. Alexandra notes that Frank is jealous of Marie because "everybody likes her, just the same as when she was little." The same magnetic qualities that attracted Frank to Marie also attract other people's attention. What Alexandra does not notice is that some people's pleasant interactions with Marie might bother Frank more than others'. Alexandra mentions that the primary time and place for her to witness Frank's "sulking" is when she goes to the Catholic church with Emil. Alexandra has always turned a blind eye to her brother's flirtation with Marie, but it seems quite possible that Frank's "sulking" is exacerbated by Emil's presence and the attention he and Marie are paying to one another. When Frank eventually kills Emil and Marie, he is acting on impulsive rage. Still, this passage suggests that his rage has had years to build into a simmer just below the surface.
In Part 2, Chapter 5, Carl watches as Emil and Marie hunt ducks. The narrator uses imagery to foreshadow Marie and Emil's violent, disappointing end:
When [Emil] came back, dangling the ducks by their feet, Marie held her apron and he dropped them into it. As she stood looking down at them, her face changed. She took up one of the birds, a rumpled ball of feathers with the blood dripping slowly from its mouth, and looked at the live color that still burned on its plumage.
Emil and Marie "laughed delightedly" when Emil first shot the ducks, but Marie is horrified when Emil brings her the dead birds. Her sudden change in attitude seems to be the result of realizing the full, bloody impact of the sport she was enjoying with Emil. The two of them have been skirting the line of propriety given the fact that Marie is married to someone else. Although they could reasonably pass off the duck hunting as a friendly game, it also seems like a game of courtship. Especially the way Emil places the duck meat in Marie's apron suggests that they are playing house: Emil retrieves meat to feed the family, and he gives it to his "wife" to cook.
Once Emil crosses this line, making their "friendly" game more difficult to distinguish from courtship, Marie takes in the up-close gruesomeness of what they have just done. The bird she picks up still has signs of recent life, but the bullet has turned it into a bloody "ball of feathers." Marie and Emil got caught up in their game and, consequently, they harmed other beings beyond repair. This moment foreshadows the couple's tragic end in two ways. First, the upsetting image of a bird suddenly killed by a thoughtless bullet foreshadows Emil and Marie's own murders. Although Frank shoots them out of rage, his temper is no less thoughtless than the delight that leads Emil and Marie to kill the ducks.
The moment also foreshadows the harm Emil and Marie inflict on others because, once again, they allow themselves to get caught up in the game of their courtship. They may end up dead, but the novel dedicates the last part to Alexandra and Frank's suffering. Frank would never have become a murderer if Emil and Marie had stopped to think about the impact of their infidelity (or so the novel suggests). Meanwhile, Alexandra would not have lost her beloved brother if he had been more cautious.
