The Spanish Tragedy is clearly a revenge play, with Revenge even being a character in the play. A question that has long been debated concerning the play and outside of the context of the play is whether Hiernonimo is morally just in seeking revenge. One can argue that Hiernomino really had no choice in seeking revenge for his son’s death, since the court would not offer him any help, granted his class status (another theme of the play).
We see Hieronimo’s inner conflict in deciding if he should choose “this way or that way” in Act III, scene xii, when Hiernomino grapples with the options of either taking his own life, or taking the way of revenge and justice for his son’s death. We see the negative connotations that are ascribed to taking the matter of revenge in one’s own hands in Hieronimo’s soliloquy, where he describes the path of revenge as a path through hell. Horatio cannot simply take these matters to the king since there are several obstacles in his way, such as his social class and diplomacy. Lorenzo is nephew of the king of Spain and Balthazar is a prince of Portugal who is a key figure in the negotiation between the two nations, so the possibility of Hieronimo finding justice by the way of the king seems unlikely.
In Act III, scene xiii, again we see Hieronimo struggling with the matter of vengeance. He exclaims, “Vindicta mihi!” or “vengeance is mine,” and considers leaving the matter of revenge to God. However, he comes to the conclusion that it is his destiny to find his revenge against his son’s murderers.
Hieronimo is a Knight-Marshal, hence carrying out justice is a part of his job. Although committing murder is an un-Christian thing to do, it seems to be Hieronimo’s only choice in a society where you cannot depend on the Crown for justice.
The classical influence in this play is so strong that there’s actually very little sense that we’re in a Christian world. Maybe the threat of burning at the stake is the closest that The Spanish Tragedy comes to a recognizably Christian image, although of course “Vengeance is mine” is biblical. See Romans 12:19, with other scriptural analogues listed on this site. http://bible.cc/romans/12-19.htm