Shakespeare uses Mercutio to serve as an intense statement for precisely what not to do. Mercutio does indeed serve as a dramatic foil for Romeo in that Romeo is deeply serious, especially about love, while Mercutio treats everything as a joking matter. However, both characters share impetuous and rash behavior in common. Shakespeare especially takes Mercutio's impetuous behavior to a high level when we see Mercutio irrationally challenge Tybalt when Romeo tries to end the fight, which of course culminates in Mercutio's death. Through Mercutio's death, Shakespeare shows us that impetuous, rash behavior is dangerous and, instead, promotes calm, rational thought above such behavior.
Mercutio's name is especially interesting because it is symbolic. Not only does it refer to the Roman messenger god, Mercury, it also refers to the poisonous, easily changeable metal that can especially be symbolic of a hot temper("Romeo and Juliet: Mercutio (Character Analysis)," eNotes). We especially see Mercutio's name being symbolized by mercury in his death scene, Act 3, Scene 1. Mercutio has absolutely no reason for remaining on the street that day, and Benvolio begs him to go inside with him, warning that, "The day is hot, the Capulet's abroad. / And if we meet, we shall not scape a brawl" (III.i.2-3). We can speculate that the only reason why Mercutio is out on the street is because, knowing that Tybalt has challenged Romeo to a dual, Mercutio is hoping to start a fight with Tybalt, which is of course a very rash and impetuous decision. Mercutio's attitude towards Tybalt's challenge explains why Mercutio is so quick to challenge Tybalt when Romeo backs away from the fight, as we see when Mercutio proclaims Romeo's efforts at ensuring peace, "calm, dishonourable, vile submission!" (III.i.72). Again, Mercutio's reaction towards Romeo's attempt at creating peace can be called impetuous and rash.
Hence, we see that Shakespeare uses Mercutio to show how dangerous impetuous and rash behavior is. We can especially see this when we notice that Mercutio is the very first rash and impetuous character to die.
Mercutio has a dual purpose in this play - he exists as both an opposite to Romeo and as a character who has the same flaws as Romeo. So how can he be both?
First, Romeo is a lovesick character. He becomes too obsessed with his own emotions and ideals. Rather than accepting Rosaline's rejection and moving on, Romeo sinks into a depression and dwells on his unhappiness. It is this tendency to get swept up in his romanticism that leads Romeo into a hasty marriage with Juliet as well.
In contrast, however, Mercutio is a lighthearted character. He lives moment to moment, enjoying life for what it puts forth, and not obsessing about what he can or can not have. He treats relationships with woman in a physical and not emotional manner. He ridicules dreamers like Romeo in his Queen Mab speech, for dreams "are the children of an idle brain,
Born of nothing but vain fantasy."
However, both Mercutio and Romeo are victims of a similar character trait - they act rashly, without thinking. Mercutio is too quick to engage in a fight with Tybalt, which results in his own death. He blames the Montagues and Capulets ("a plague on both your houses!"), but really it is his own fault. He did not have to get involved. Romeo also dies as a result of acting too quickly. Had he waited for more information, he would have learned that Juliet wasn't dead, and thus not have killed himself.
He is the friendly (meaning not openly hostile, like Tybalt) opposition to Romeo. Mercutio's character never hesitates to act in opposition, if for no other reason, to get Romeo to think about his actions...especially when it comes to love and loyalty.
Mercutio knows how much of a romantic that Romeo is, so when he is heartbroken over Rosaline, he uses the Queen Mab story to begin to distract him, discussing love as both the emotion we love and the thing that keeps Romeo (and many others) from being truly happy.
Then, as Mercutio dies, he seriously questions Romeo's loyalty, asking, "Why the devil you came between us?" Romeo thinks he is doing the right thing, but Mercutio's death reminds him that he is only thinking from his lover's perspective, and choosing to ignore the giant family feud that he is a part of, like it or not.
Mercutio's main purpose is to act as a foil to Romeo. A foil is a character who, when contrasted to another, clarifies a character's personality. For much of the play, Mercutio is a lighthearted jokester who trivializes love, reducing it to sexual innuendo through his witty use of puns. Romeo, on the other hand, is serious all of the time and lovesick.
Mercutio is more hot tempered when Tybalt seeks out Romeo. When Romeo refuses to fight, Mercution fights instead, even when Romeo tells him to stop. Mercutio's death sets up the ironic twist in the climax as Tybalt stabs Mercutio under Romeo's arm. Mercutio foreshadows the end of the play when he says, "A plague o' both your houses!"
In the play, after Tybalt stabs him, he curses both houses. This is a HUGE foreshadowing event that takes place. He also tends to be the voice of reason with trying to get Romeo over Rosaline in the beginning and making him go to the Capulet ball. One could, loosely, say that because of Mercutio, Romeo and Juliet actually met.
When Mercutio gets stabbed, Romeo also hits the Alpha-Male stage and decides to avenge his death and man-up as it were. Before, he was happy being in love. Because of this killing and Romeo's retaliation, the events that led up to the death of the two young lovers started to play out.
Mercutio plays the part of the tension-releaser. He is the dreamer, the deliverer of the speech about Queen Mab, the jokester with regards to Romeo's "fall in love quick" schemes. He lightens the mood and moves the plot forward when he is stabbed under Romeo's arm by Tybalt. Even in death Mercutio is a drama King and takes a more dramatic, longer time to die than any other character. He makes us laugh in the face of many disasters and tensions.
Mercutio is the "class clown," the king's jester or the comedian who entertains the audience during intermission. Mercutio's death is the spark that re-ignites the age-old battle between the two families, yet, ironically it is Romeo's fault that Mercutio gets killed. Shakespeare had to kill off Mercutio. He is, by far, the most interesting character in the play. If he were to live, we would be more interested in what he had to say and do than in the story of Romeo & Juliet. Also, despite all his jokes, he was by far the wisest and most intelligent of the characters. I wonder if Shakespeare didn't see a bit of himself in Mercutio. I especially love his character in the Zeferelli movie.