In Shakespeare's Romeo and Juliet, in Act III, scene i, I think that Mercutio is something of a show-off, and has a lot to say to mock others, even Benvolio. I don't think that his intent is malicious: he is just someone who doesn't take too much seriously and loves to stir things up a little. However, when Tybalt comes along, a member of a noble house, and is rude, Mercutio takes great offense. He is not someone to be easily pushed around. He also get annoyed when Romeo will not engage in trading insults with Tybalt. (Of course, Mercuitio does not know that Romeo is now married to a Capulet, but thinks Romeo is simply acting cowardly.)

Returning Tybalt's disrespect, Mercutio insults him:

Tybalt, you rat-catcher, will you walk? [step aside]...

And then when Tybalt asks what Mercutio wants with him, Mercutio responds with another insult or two:

Good King of Cats, nothing but one of your nine lives that I mean to make bold withal, and as you shall use me hereafter, dry-beat the rest of the eight. Will you pluck your sword out of his pilcher by the ears? Make haste, lest mine be about your ears ere it be out.

Mercutio has already called Tybalt a "rat-catcher," and now he carries on the comparison stating he wishes to take one of this cat's proverbial "nine lives," and if Tybalt continues to insult Mercutio, Mercutio will beat up the remaining eight. Then he challenges Tybalt to draw his sword: quickly or Mercutio will cut off his ears. Tybalt draws and they commence to fight.

Tybalt is a hot-head, and he hates the Montagues with a passion: not because he agrees with their dispute—as far as we know, neither Capulet or Montague remember what this age-old feud is about. But Tybalt needs little reason for stirring things up; he is a trouble-maker. And Mercutio, despite his biting wit and over-the-top sense of humor, has no time for this blackguard (scoundrel). He doesn't have much time for the Capulets, either, but for him it's more personal: based on how Tybalt directly treats Mercutio simply because he is friends with Romeo.

It is under these circumstances that Tybalt reaches around Romeo's body, as he tries to stop the fight, to take a cheap shot at Mercutio that eventually brings about the young man's death—as he is dying, Tybalt has run away.