In Act I, Scene iii of The Merchant of Venice, explain the stanza, " How like a fawning publican he looks! I hate him for he is a Christian; But more, for that in low simplicity ..... Which he calls interest. Cursed be my tribe, if I forgive him!
One of the main contentious issues in The Merchant of Venice, is the hatred between Shylock and Antonio. Each treats the other contemptuously. In this scene, Shylock and Bassanio have just made an agreement for Antonio to obtain a loan and now they will meet to discuss the terms. Shylock, aside to the audience, makes it clear how he feels about him.
He is like "a fawning publican" a grovelling tax collector. Although a publican is traditionally someone who owns a bar or a tavern as it would have been called in those days, in ancient times a publican was the hated and much-aligned collector of taxes so Shylock is comparing Antonio to that. Shylock goes on to say "but more for that.." his contempt goes much further. "He is a Christian." He also lends money to others "gratis;" in other words, free of any charges or interest. The "rate of usance" is the rate at which money lenders are allowed to claim interest on the loans.
Shylock looks forward to an opportunity to "catch him once upon the hip" meaning he wants to catch Antonio out, allowing him to "feed fat," take advantage, really benefit, satisfying "the ancient grudge." Antonio hates "our sacred nation" meaning the Jewish people and he constantly "rails" or criticizes Shylock and his business interests ("my bargains") and his success ("my well-won thrift) in public places where other merchants meet.
If Shylock forgives Antonio for being so disrespectful towards Jews, he feels that he would be doing the Jews "my tribe" a disservice and they would be "cursed," never able to rise above their hated status.