How does revenge in The Merchant of Venice relate to the present day?

Revenge is a timeless subject; the desire to exact vengeance on someone who has done you wrong has existed for as long as humans have, and the desire for revenge in The Merchant of Venice is still as poignant today as it was when the play was written.
The central vengeance in the plot focuses on Shylock the Jew getting revenge on Antonio, who is both a competitor and also a Christian. Shylock is an outsider in his society, and although he is written by Shakespeare to be a loathsome villain, many modern commentators have seen him in a more empathetic light. Jews in medieval and Renaissance Europe were the only ones who could lend money and receive interest because Christians were not allowed to participate in usury. Usury was one of the main reasons that people in Europe despised and mistreated the Jews who lived there, and it is one of the leading social crimes that Shylock commits in the play.
The revenge that Shylock seeks is not justified in the play, because it is explicitly leveled at Antonio. The reality is that Shylock is justified in how he feels, to some extent. During the trial at the end of the play, Portia explains that Jews are outsiders in their society and are not given the same rights as citizens:

It is enacted in the laws of Venice,If it be proved against an alienThat by direct or indirect attemptsHe seek the life of any citizen,The party 'gainst the which he doth contriveShall seize one half his goods; the other halfComes to the privy coffer of the state;And the offender's life lies in the mercyOf the duke only, 'gainst all other voice. (act 4, scene 1)

Shylock has no real power in Venice. He is not a citizen, and he doesn't have the right to seek the life of a citizen. His religion and race are used against him, and he stands before the law at the end of the play without any real protection. Ultimately, his desire for revenge is his undoing.
Shylock, in seeking the life of Antonio, has misplaced his anger. He goes too far in trying to attack just one person, because Antonio is not the cause of his problems. Antonio is a perfect stand-in for the society that oppresses Shylock because he is both a Christian and someone who loans money without interest. However, the revenge that Shylock takes doesn't solve his problem, and it does nothing to create real justice.
The situation of Shylock is similar to many of the atrocities carried out by isolated or oppressed groups in the world today. Groups and people attempt to cause social change through violence in the world. That violence often takes the form of acts of revenge against people who the perpetrators believe have wronged them. Those acts of violence, usually against innocent people, don't bring about societal change, and they don't bring justice: but people still do it because their anger is misplaced.
Shylock is similar in the way he takes revenge. He seeks to take revenge against someone that has done nothing wrong. That is why Shylock is considered a villain—because his actions and aggression are misplaced. How he feels can be understood; he is persecuted for his job, his race, and his religion in the play, but the actions he takes for vengeance are not justifiable.