What old grudge does Shylock have against Antonio ?

Both Antonio and Shylock have had business dealings in the past. Shylock claims that Antonio has spit on him, called him a dog, and insulted him for lending money for profit. Shylock feels persecuted by Christians and by Antonio in particular.

Shylock bears a grudge against Antonio because Antonio has been bringing down the rate of interest in Venice by lending out money without imposing interest. Shylock is also hostile to Antonio because Antonio hates the Jews and because Antonio has been criticizing Shylock at the Venetian Stock Exchange. Antonio has been condemning Shylock's money-lending operations and the undue profits which Shylock has been making from those operations.