In this speech, Shylock shows two sides to his personality. He shows that he is sensitive and is hurt by the prejudices that people have against him because he is a Jew. At the same time, he shows that he is angry and is willing to hit back at others.
He shows the first of these sides with the more famous part of the speech. This is the part about how Jews are just the same as anyone else. He shows the second at the beginning and end of the speech. He talks about using the pound of flesh to bait fish hooks and he talks about how badly he wants to get revenge on Antonio.
In Act III Sc.1 Shylock pours out his venom and hatred towards Antonio to whom he had lent a large sum of money and is now unable to repay the amount because of the loss of his ships. The two Venetian gentlemen Salanio and Salarino inquire about Antonio's misfortunes and at once Shylock tells them that the now bankrupt Antonio must fulfill the conditions that he had laid down in the bond when he borrowed money from him. The condition was that if Antonio was not able to repay the money Shylock would take a pound of flesh from him:
Salarino asks him what's the use of Antonio's flesh to Shylock. At once Shylock explodes passionately expressing his hatred against Antonio and all the Christians who have persecuted him all these days.
The fact of the matter is that Salarino and Salanio have chosen the wrong moment to prompt Shylock to be generous and condone Antonio's inability to repay the money that he has borrowed from him -- Shylock is furious because his daughter Nerissa has just eloped with the young Christian gentleman Lorenzo taking away with her all her father's money. The irony being that the Salarino and Salanio do not know about this and hence they are at the receiving end of Shylock's hatred and scorn for all Christians.
Shylock's hatred for Christians is best summed up in these lines:
If a Jew wrong a Christian,
what is his humility? Revenge. If a Christian
wrong a Jew, what should his sufferance be by
Christian example? Why, revenge. The villany you
teach me, I will execute, and it shall go hard but I
will better the instruction.
Criticism of Shylock's paranoid hatred against the Christians and his keen desire to avenge the ill treatment meted out to him by them should be tempered with the realization that there is a lot of truth in what he says and the realization that he is an angry father whose daughter has just then run away with all his money.