The Perfect Match by Jodi Picoult
Assistant DA Nina Frost prosecutes child molesters, and in the course of her everyday work she endures the frustration of seeing too many criminals slip through the system and walk free. So when she realises that her son Nathaniel has been sexually abused - and is so traumatised that he has stopped speaking - she takes justice into her own hands. Nina Frost may have killed the man who hurt her son, but has she destroyed her family in the process? And whatever happened to innocent until proven guilty?