Justify Shylock's demand for revenge against Antonio.

In the very first scene that Shylock and Antonio appear
together in The Merchant of Venice, Shylock says that in the past,
Antonio has treated him cruelly because Shylock is Jewish. Shylock says Antonio called
him a

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misbeliever, cut-throat, dog / And spit upon my
Jewish gaberdine.
(I.iii)

Shylock also says
that Antonio kicked him on the street like a stray
dog:

[You]
foot me as you spurn a stranger cur

Over your threshold!
(I.iii)

I don't know about
you, but being called cruel names and being spit at would make me pretty
upset!

We know that what Shylock says is true, because
Antonio himself confirms it. Rather than denying the name-calling and spitting, he
agrees that he's done it many times before and will continue to do so. He is not
appealing to Shylock to lend him money as a friend, but as an
enemy.