I think her gut reaction was the same that most of us would have. I couldn't imagine being in Sam's position of losing my family, being accused, etc.... but then for him to go above and beyond to forgive the killer, it's emotional stuff. I think even the most faithful of people would have a struggle with that type of forgiveness. When Jesus died he forgave those who persecuted him but the apostles had a hard time with that concept. In my daily life I typically forgive the wrongs that have been done to me because life is just too short to carry that around. However, I'm forgiving stupid things in comparision to what Sam forgave. I'm inclined to believe I would react in much the way Rachel reacted.
I think Rachel's reaction was a very human reaction. She was not only observing what Sam had been and was still going through, but her life had also been affected. Despite hearing over and over that God forgives us, that holding a grudge only hurts us, that forgiveness frees us..... it is still hard to do in extreme cases like this.
I think a lot of Rachel's reaction probably had a lot to do with her past also. We didn't get more than a quick glimpse, but knowing that her brother killed himself while serving for a crime he didn't commit makes me think that her reaction was based on feelings leftover from that tragedy. Sam is the second loved one she's watched go through this type of horror and though he made it out alive unlike her brother, he suffered; suffered exponentially and I think that makes it very hard for her, despite her strong faith, to forgive the guilty party.