I just saw Atonement today, and am left with lots of impressions.
I personally enjoy novels or films where the narrator/point of view shifts or is ambiguous and this is certainly the case here.
Because I was very emotionally affected, and a friend I saw it with was not, I had to think of the "universal". For me, it was the "thou shalt not bear false witness" thing.
I think the challenge for the audience is: can you feel sympathy for Briony? (This is not the same as forgiveness.)
My sympathy for her came when she told the French soldier, who was obviously going to die, that she loved him. Perhaps her first completely selfless act (and, perhaps, also a lie). To me, she suddenly became human, with his blood on her face.
Clearly, as a girl, she was in love with Robbie; I'm sure she was confused by the sexual images she witnessed between him and Cee. Don't forget, 13 year olds can have intense sexual feelings. One can see how her jealousy led to her lie, which takes him away from her sister.
Of course, we have to think about the "atonement" of the title: she has to live with the guilt and widespread effects of her actions for the next 60+ years.
As a side note, I found the "mole" on her right cheek a physical representation of her "sin". Notice how it was continued in all three actresses.
Finally, the act of writing the novel, at least to the author, helps redeem her (boy, I love Vanessa Redgrave). She has given them a life together, their happiness by the "White Cliffs of Dover".