Symbolism, Imagery, Allegory
None of the murders in the book are actually in the book, precisely. That is, Pat and Robert both committed murders, but Prejean wasn't there when they happened, and she doesn't see the aftermath directly. The murders are all off screen; they occurred before Prejean showed up.
So, since the murders are not really part of the narrative, you wouldn't necessarily need to hear about them, or have them described. But the book nevertheless goes out of its way to include details of the deaths, which are sometimes extended, and sometimes just a sentence or two. Prejean is talking to Pat about what he did, and she says "I see the young people getting down on their knees and lying down in the cold wet grass" (2.120). Or at the very end of the novel she talks about David, who Pat killed, and how his father Lloyd thinks about what he might have been: "David at twenty, David at twenty-five, David getting married, David standing at the back door with his little ones clustered around his knees, grown-up David, a man like himself, whom he will never know" (11.157).
The inclusion of the murder victims is important in terms of both the moral and the message of the book. Prejean argues that the death penalty is wrong, but that argument is based on the idea that all killing is wrong. That means that the murders are wrong too—and if the killing of Pat and Robert is important enough to dwell on, then the killing of Faith and David and Loretta have to be important enough to dwell on too. The murder victims are included to show that every death matters, and every life is precious.