Shylock hates Antonio because Antonio has the privilege of being a wealthy Venetian who charges no interest on his loans, and he also hates Antonio for being a Christian. Additionally, Shylock hates Antonio for the outspoken disdain that Antonio displays towards him.
Early in act I, Shylock expresses his hatred for Antonio.
I hate him for he is a Christian. . .
He lends out money gratis and brings down
The rate of usance here with us in Venice.
If I can catch him once upon the hip,
I will feed fat the ancient grudge I bear him. (1.3.34-38)
Antonio not only loans money interest-free to many, he has also covered the loans of Shylock's victims without charging them interest to repay him. This action has enabled Shylock's victims to escape total ruin, as they pay off their loans to Shylock quickly without having to pay most of the added charges for these loans. Then, they can repay Antonio simply for the amount of their loan. Furthermore, Antonio's action also undercuts Shylock and forces him to lower his interest rates in order to get others to borrow from him.
In addition to undercutting Shylock, Antonio has denounced Shylock in public, calling him a dog. He has even kicked and spat upon Shylock. Shylock reminds Antonio of these insults:
In the Rialto you have [be]rated me
About my moneys and my usances.
Still have I borne it with a patient shrug. . .
You call me misbeliever, cutthroat dog,
And spet upon my Jewish gaberdine—
And all for use of that which is mine own. (1.3.105-111)
Antonio's insults help motivate Shylock to loan Antonio money with a horrific penalty for failure to make repayment. If Antonio fails to repay Shylock, Shylock can exact a brutal revenge against his enemy.
Shylock would hate Christians in general since, at the time of this play, Jews in Venice were made to live in a ghetto and were prohibited from being in Venetian society. The word ghetto may have been derived from the Italian word, getto, meaning foundry. (The first ghetto was established on the site of a foundry in Venice in 1516), or from the Italian word borghetto, which is the diminutive of the word borgo that means borough.