As we've found across The Merchant of Venice, Shylock is driven primarily by the desire for revenge. He wants retribution for the pains and humiliations that Christians have visited upon him. This is the essential message of the "I am a Jew" speech. Then, act 4, scene 1 is the courtroom scene where Shylock presses his claim against Antonio, and we see the themes of mercy and justice come fully to the forefront.
Early in this scene, after Shylock enters, the Duke makes a plea for Shylock to show mercy to Antonio. As he says to Shylock:
Shylock, the world thinks, and I think so too, / That thou but lead'st this fashion of thy malice / To the last hour of act; and then, 'tis thought/ Thou'lt show thy mercy and remorse more strange / Than is thy strange apparent cruelty; / And where thou now exact'st the penalty, / Which is a pound of this poor merchant's flesh, / Thou wilt not only loose the forefeiture, / But touch'd with human gentleness and love, / Forgive a moiety of the principal.
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