After reading the play, you might think Hieronimo is just a bloodthirsty avenger hell-bent on killing and satisfying his rage. Fair enough. he does eventually become insanely murderous. But he didn't start the play in love with death and mayhem.
So let's back up a bit. To understand Hieronimo, let's talk a bit about his day job, in which he served as the Knight Marshall of Spain.
As Knight Marshall, Hieronimo is the chief judge of legal matters in Spain. This guy is a Big Deal, job-wise, because he's in charge of dealing with the King's most important legal issues. It's part of his day-to-day work to bring justice down on murderers. And this becomes very real to us as he serves a verbal beatdown on a defiant murderer awaiting his death on the gallows:
Peace, impudent! For thou shall find it so.
For blood with blood shall, while I sit as judge…
Dispatch! The fault's approved and confessed,
And by our law he is condemned to die. (3.6.34-40)
Pretty heavy justice, right? The 16th-century legal system demanded blood for blood, period. So it really isn't a stretch for Hieronimo to go all Rambo when the justice system fails his family. Need some help with the Rambo reference? Let's indulge. Ahhh, the 80's.
Anywho, Hieronimo isn't just some crazed civilian with an overinflated sense of justice. No, he is justice (or, should be). We aren't trying to say his job made him do it. It's just that his job helps us to see the conflict between revenge and justice in the play. So to understand Hieronimo is to begin to understand how the pursuit of justice can quickly degrade into revenge and criminality. This is maybe the whole point of the play, so you'll want to pay close attention to how his pursuit of revenge impacts his character.
The dedication and drive Hieronimo brings to his professional life also defines his role as private avenger. From the moment he discovers his son's murdered body, Hieronimo commits himself to bringing the baddies to justice. He expresses his commitment to aggressive justice by making these vows over his son's dead body:
Sees't thou those wounds that yet are bleeding fresh?
I'll not entomb them till I have revenged.
Then will I joy amidst my discontent;
Till then my sorrow never shall be spent. (2.5.53-56)
Yes, deciding not to bury his son is literally a stinky decision. But this decision also tells us that Hieronimo is an extremely committed dude. But here we also get our first glimpse at how emotional Hieronimo can be. He will indeed feed his sorrow before he gets revenge, and the high-flying emotions he expresses in the process make him an extraordinary character for the period.
We tend to take emotionally complex characters for granted in Renaissance plays. But it's important to remember that characters were pretty shallow before Shakespeare began churning out the realness some twenty years later. Hieronimo is a prototype for the kind of violent, emotionally conflicted avengers that still entertain us today. Just ask HBO.
So yeah, we have Hieronimo to thank for inspiring Shakespeare to write Hamlet. But what about, say, The Incredible Hulk? He's also an avenger driven by anger, bad memories, and painful emotions. And this kind of hero started with Hieronimo. Think about it. Hieronimo smash.
But the really revolutionary thing about Hieronimo is how he changes throughout the play. And again, we have to appreciate what most characters were like before Shakespeare forever changed dramatic characterization. They were, well, boring and unchanging. In fact, Hieronimo is the only character in The Spanish Tragedy to experience any growing pains at all.
The whole evolving character thing is actually an invention, and Hieronimo is definitely one of the first dynamic characters in the history of drama. Prior to Hieronimo, playwrights for the most part created character types (good, evil, comic, tragic, etc.) that they would simply place in different contexts without much inner change.
That means that Hieronimo's complexity is a literary breakthrough.
So how does he change? Well, as it becomes clear that being a good guy won't get him anywhere, he breaks bad, so to speak. In other words, think Walter White in Breaking Bad while wrapping your head around how Hieronimo is changed by the dirty business of revenge. Of course Walter only gets into making drugs to save the family home, but along the way he does some really bad stuff—including, um, making drugs.
Hieronimo is the same: he's thinking of his family while he kills, but still he kills. And he even goes the extra mile to gut a character that has zero to do with the death of his son. Hieronimo is well aware that he has crossed over to the dark side when he says:
Per scelus semper tutum est sceleribus iter
Strike, and strike home, where wrong is offered thee,
For evil unto ills conductors be. (3.13.1-9)
What? You don't read ancient Latin? Total slacker.
Fine, the first line means, "Vengeance is mine," and the second one reads, "The safe way for crime is always through crime." By claiming vengeance as his own and declaring that the only way to punish crime is through crime, Hieronimo, the man of law, has ventured into the world of criminality and frontier justice.
The classic question about Hieronimo is whether or not he is actually crazy. One thing is for sure: he acts crazy.
But that's just it: Are his antics just an act? He does come out and say, "Thus therefore will I rest me in unrest,/ Dissembling quiet in unquietness," which is just an Elizabethan way of saying, "I'm going to act restless and crazy so nobody knows what I'm up to." Do we take him at his word? Or has breaking bad really driven him nuts?
We'll leave this up to you to decide. It's hard to say definitively one way or the other. But you can take a side and make your argument. Yes, we're talking to you. So, start looking for textual evidence and make your claim. And then tackle the very same problem in Hamlet. Get ambitious.