The Mercy Clause by Philip Braithwaite

Tom is about to take on the case of his career: a young man named Brian is accused of killing his father. The defense should be straightforward: it was a mercy killing. Brian however is anything but straight forward, striking up a connection with Tom's wife Rachel, telling her parts of his story that differ from what he's told Tom. As Tom and Rachel work out Brian's true story, the truth in their marriage is revealed.