SHYLOCK. If I can catch him once upon the hip,
I will feed fat the ancient grudge I bear him. (1.3.41–42)
With these two lines, Shylock sums up his feelings towards Antonio and towards everyone else who has wronged him, individually and collectively, throughout his life. If Shylock can get an advantage over Antonio, he will take his revenge against him without a second thought, and without remorse.
Within minutes, Shylock makes a contract for a loan to Antonio, the enforcement of which sets Shylock's revenge into motion and which destroys one of them and almost destroys the other.
Shylock feels perfectly justified in seeking his revenge against Antonio when the loan comes due and Antonio is unable to pay Shylock what he owes.
SHYLOCK. He [Antonio] hath disgraced me, and hindered me half a million; laughed at my losses, mocked at my gains, scorned my nation, thwarted my bargains, cooled my friends, heated mine enemies; and what's his reason? I am a Jew: hath not a Jew eyes? Hath not a Jew hands, organs, dimensions, senses, affections, passions? Fed with the same food, hurt with the same weapons, subject to the same diseases, healed by the same means, warmed and cooled by the same winter and summer, as a Christian is? If you prick us, do we not bleed? If you tickle us, do we not laugh? If you poison us, do we not die? And if you wrong us, shall we not revenge? If we are like you in the rest, we will resemble you in that. 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 villainy you teach me I will execute; and it shall go hard but I will better the instruction. (3.1.47–64)
For all the talk of mercy in the play, precious little mercy is actually shown to those most in need of it. Shylock refuses to show any mercy to Antonio for defaulting on his loan. Antonio now owes Shylock a pound of his "fair flesh," which Shylock is determined to collect.
DUKE. Therefore, Jew,
Though justice be thy plea, consider this—
That in the course of justice, none of us
Should see salvation: we do pray for mercy;
And that same prayer, doth teach us all to render
The deeds of mercy. I have spoke thus much,
To mitigate the justice of thy plea,
Which if thou follow, this strict court of Venice
Must needs give sentence 'gainst the merchant there.
SHYLOCK. My deeds upon my head! I crave the law,
The penalty and forfeit of my bond. (4.1.200–210)
PORTIA. Why, this bond is forfeit;
And lawfully by this the Jew may claim
A pound of flesh, to be by him cut off
Nearest the merchant's heart.—Be merciful;
Take thrice thy money; bid me tear the bond.
SHYLOCK. When it is paid according to the tenor.
It doth appear you are a worthy judge;
You know the law, your exposition
Hath been most sound; I charge you by the law,
Whereof you are a well-deserving pillar,
Proceed to judgment: by my soul I swear
There is no power in the tongue of man
To alter me: I stay here on my bond. (4.1.234–246)
Precious little mercy is shown to Shylock when the judgment goes against him. Only the Duke appears to show any mercy towards Shylock, by pardoning him from his death sentence. To Shylock, though, this is a living death, which hardly qualifies as mercy.
In the name of "mercy," Antonio makes demands of Shylock that will destroy him.
PORTIA. What mercy can you render him, Antonio? ...
ANTONIO. So please my lord the duke, and all the court
To quit the fine for one half of his goods;
I am content, so he will let me have
The other half in use, to render it,
Upon his death, unto the gentleman
That lately stole his daughter;
Two things provided more,—that for this favour,
He presently become a Christian;
The other, that he do record a gift,
Here in the court, of all he dies possess'd,
Unto his son Lorenzo and his daughter. (4.1.390–402)
Even when the court rules against him, everything of consequence is taken from him, and he's forced to become a Christian, Shylock doesn't ask for mercy. It might be that, based on the experience of his life, Shylock knows that it would be a waste of time to ask for mercy from the Christians who are judging, punishing, and taking their own merciless revenge against him.