The Question and Answer section for Atonement is a great resource to ask questions, find answers, and discuss the novel.
In my opinion, Briony is a criminal. She knowingly accuses Robbie of rape merely because of what she has witnessed between Robbie and her sister. This is, of course, my opinion.
Paul Marshall was the man who raped Lola. Nothing happened to him because he was never tried, punished, or even accused. He remained free to go on with his life because someone else paid fro his crime.
The first crime is the assault and rape of Lola, who has recently arrived at the Tallis home.
The second crime is Briony's lie, the accusation she makes against Robbie as the man who raped Lola..... a false accusation.