In Act IV scene I, Portia not only releases Antonio from his bond but forces Shylock to give up his case entirely. Portia explains that because the bond did not specify Antonio owing an ounce of his blood, if blood is spilled by Shylock's hand, he will be guilty of conspiring against the life of a Venetian citizen. She goes further by saying that because Shylock is not legally a citizen, he faces a penalty for threatening the life of a Venetian citizen. Half of his estate must go to Antonio and half must go to the state. The state grants him mercy by not demanding him to pay them half of his estate, only a fine. Antonio then offers to return his half on the condition that Shylock convert to Christianity and give his daughter and her husband all he possesses upon his death.
I have consistently felt bad for Shylock throughout the play. He is made out to be the villain and yet he is humiliated and cut down at every turn by the supposed "heroes" of the play. This is a man who is not even considered a citizen because of his religion. A man who has been kicked spit upon and publicly humiliated by the Christians of the community. His daughter has run off with one of his tormentors, renounced her father's religion and stolen from her father. All Shylock has in the world is his money. It gives him the power to hold Antonio and people like him as he does. People come to him to borrow money and in that way he has some kind of control, some kind of power in a world where he otherwise is and has nothing. Of course he wants justice, of course he wants to exact his revenge on Antonio when given the chance. Shylock is angry. He's desperate. He wants to show people that he can not be pushed around forever and treated like a dog forever. He wants to prove that at some point, one of them will slip up and he will be able to avenge himself and he'll do it perfectly within the bounds of the law. This moment, where he takes revenge on Antonio was to be, probably the one time in his life where he saw any justice.
Even this moment was taken from him. He was not even allowed one moment of satisfaction. He was teased with the idea that he might possibly kill one of his Christian abusers and then it was swiftly taken away from him. They proved that he will not ever be in control. At any moment they might take all that he holds dear in the world and no one will ever side with him.
Although he does not lose his entire estate, he is stripped of his religion, his identity. And he must give his fortune, upon his death, to his daughter who betrayed him. Why kind of justice is that for a man? He has no say in his fate. He has no say in the goings on in his life. He thought that his money wielded some kind of power for him and in one moment, when he tried to truly exercise that power it was ripped from him and he was shown just how weak he really is. "I pray you give me leave to go from hence. I am not well," he says in line 391 of the first scene of Act IV. He returns to his home, shamed and defeated while his enemies rejoice at having shut him down once again.
Shylock is not the villain in this play. Shylock is the poor, downtrodden nothing that gets no justice and no redemption. He is doomed to endure the beatings and the public shaming and now, after his case with Antonio is closed, he is forced to endure all of this knowing that there is truly nothing he will ever be able to do about any of it.