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.

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.

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.

It is clear that there is little love lost on both sides of this relationship. Clearly Shakespeare intends Shylock to be a representative of the Jews, and likewise, to Shylock, Antonio becomes a representative of Christianity and the way that it has consistently mistreated Jews down through the ages. Note what Antonio says to Bassanio after Shylock tells the story about Jacob:

The devil can cite Scripture for his purpose.

An evil soul producing holy witness

Is like a villain with a smiling cheek...

He goes on to say to Shylock that he should lend the money to "thine enemy" rather than to his friend.

Shylock is even more overt about his feelings towards Antonio. Note how, as he watches Antonio draw near, he says, in an aside:

I hate him for he is a Christian.

He plans immediately to "feel fat the ancient grudge I bear him" and somehow to get his own back for the way that he lends money without charging interest and also for the personal wrongs that Antonio has committed against him.

In this scene, therefore, Shakespeare introduces us to the main conflict of the play and we can see its roots and its development in the "merry bond" that Shylock proposes and Antonio accepts.