Shakespeare uses The Merchant of Venice as a tool to reveal the often confusing and contradictory nature of justice and compassion such as it exists in a Christian context. At the outset, Antonio laments his position; perhaps it is his "hopes abroad"(I.i.15) as his fortune rests on the safe return of his ships to port although he denies this and believes that maybe on the "stage, were every man must play a part" his is ".. a sad one."(76)Antonio is unable to justify his sadness but later will be in a position of control against Shylock and will cleverly manipulate the situation, all thoughts of sadness banished.
Portia enters the plot early and feels basically powerless to disobey her late father's wishes of finding a suitor for her: "I cannot choose one, nor refuse none."(23) As an intelligent, strong-willed woman, this need to submit to filial (family) loyalty is not rational but required of her. The power her father has over her is very real but Portia cleverly manages her suitors to hopefully ensure a result in her favor. Later, Portia will outwit Shylock in her handling of the bond payable by Antonio in the form of a "pound of flesh." Portia's behavior then shows her control over situations by using rational arguments.
Shylock is ecstatic that Portia, disguised as Balthazar, a lawyer who will judge the case against Antonio, will not "alter a decree established"(IV.i.214) and intends to uphold his bond of "a pound of flesh to be by him cut off." (227)He feels justified and redeemed and can barely contain his excitement at the thought of taking his bond "nearest his heart"(249) until Portia uses the same system of justice against him and, beaten he attempts to take "my principal and let me go" ( 231)which is no longer an option open to him as he "shall have nothing but the fofeiture."(338) Psychologically, Shylock's oneupmanship has been completely destroyed without any need to resort to violence or unjust measures. All is as it is meant to be.
The audience of Shakespeare's day would have been happy at the clever manipulation and would have felt vindicated against "the Jew" whom they considered a social outcast, even "a devil." Psychologically, the audience feels that it is the right outcome as it appears that the Christian values have been upheld and any attempt to interfere - from any other source - has been minimized. The audience itself would have felt empowered as Shylock was forced into submission.