Shylock's behaviour is in the old religious tradition of revenge ('an eye for an eye'), while Portia argues for Christian mercy and forgiveness. But in the great trial scene she has to win according to the law. The document allows Shylock to cut Antonio with his knife, and he chooses to do this near Antonio's heart. But it does not say anything about blood. Shylock loses because Antonio must not lose a drop of blood when he cuts him.