Antonio, in contrast to his warm and generous feelings toward Bassanio, feels hatred and contempt for Shylock's being a Jew. We learn from his conversation with Shylock that Antonio has a history of speaking in a cruel and derogatory way toward Shylock. He's even spit on Shylock. As Shylock notes:
Signor Antonio, many a time and oft
In the Rialto you have rated me
About my moneys and my usances.
Still have I borne it with a patient shrug,
For sufferance is the badge of all our tribe.
You call me misbeliever, cutthroat dog,
And spet upon my Jewish gaberdine.
Shylock also notes that Antonio has kicked him and humiliated him, as well as called him a dog.
Antonio doesn't deny Shylock's words or offer any apology for the treatment, stating that he will probably do it again:
I am as like to call thee so again,
To spet on thee again, to spurn thee too.
If thou wilt lend this money, lend it not
As to thy friends, for when did friendship take
A breed for barren metal of his friend?
But lend it rather to thine enemy,
Who, if he break, thou mayst with better face
Exact the penalty.
Antonio makes it clear that he considers Shylock an enemy and holds him in contempt for charging interest on a loan. The loan he wants from Shylock is nothing more than an unpleasant business deal.
Shylock has earlier expressed his hatred of Antonio, both for Antonio's being a Christian and for Antonio's lending money for free ("gratis"), rather than charging interest. He says that Antonio's loans lower the interest rates he can charge. He also says that he has a long-lasting grudge against Antonio:
How like a fawning publican he looks!
I hate him for he is a Christian,
But more for that in low simplicity
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.
Shylock later says he would do his whole Jewish tribe wrong if he didn't attempt to get revenge on Antonio.
In this scene, Shakespeare shows how deeply the animosity between the two men runs. We also see tribalism at work here. Antonio is kind within his own tribe of fellow Christians but takes out his aggressions in cruel ways against the Jewish Shylock, merely for being ethnically different. We can understand why Shylock would feel rage at his abuser.