What is a justification for Shylock seeking a pound of Antonio's flesh in The Merchant of Venice?
Shylock's primary motive for seeking a pound of Antonio's flesh was to take revenge. At the time that Antonio signed the bond as surety for the loan Shylock made to Bassanio, the moneylender had no idea that Antonio would later experience misfortune and lose his ships. He had, however, included this as a condition in the deal, obviously hoping that Antonio would forfeit and that he would then be able to have his revenge. The reason for Shylock seeking revenge is clearly stated in the aside he utters in Act 1, scene 1:
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.
He hates our sacred nation, and he rails,
Even there where merchants most do congregate,
On me, my bargains and my well-won thrift,
Which he calls interest. Cursed be my tribe,
If I forgive him!
Antonio had agreed to this particular condition for he was confident that his ships would all arrive safely at their destinations and that he would easily settle the debt within the three month period. He acted against Bassanio's advice to sign the bond for he was putting himself at risk. Bassanio, unlike Antonio, had the presence of mind to realize that there always existed the possibility that things might go awry.
Shylock obviously shared this sentiment for he was generous enough to exclude charging interest on the bond. As it was, Shylock had nothing to lose whilst Antonio, conversely, could pay with his life. As it turned out, fortune favored Shylock since Antonio suffered disaster when all his ships and their precious cargo were destroyed. It seems as if the Christian merchant had tempted fate too much and suffered the unfortunate consequences.
In addition, Shylock believed that the law was on his side and that his appeal for restitution was justified. Antonio was, after all, of sound body and mind when he signed their agreement. He knew exactly what the conditions were and signed the bond even against his best friend's admonition that he did not trust Shylock.
Ironically, though, it was Shylock's malice that turned against him. He was so determined in hurting Antonio that he turned a deaf ear against numerous appeals for mercy. He even rejected an offer by Bassanio of twice the original amount owed to him. Shylock believed that he was entirely within his rights. Eventually, it was Portia's intelligent intervention that not only saved Antonio's life but also irrevocably altered the Jew's destiny.
Portia proved to the court that Shylock's malicious claim against the Christian was a crime punishable by death and the forfeiture of his entire fortune. The duke, though, was merciful and spared him his life. Antonio's also appealed, and spared him the ignominy of losing his entire fortune. In the end, Shylock had to forfeit half of his estate to his daughter and her Christian husband and had to relinquish his own religion and become a Christian.