When Tybalt provocatively challenges Romeo to a fight despite Romeo's attempts at peace, Mercutio vehemently intervenes on Romeo's behalf. Incensed by Tybalt's insults and mockery, Mercutio engages in a duel with him. During the fight, he suffers a fatal wound inflicted by Tybalt. In a moment of rage and frustration, Mercutio curses both Romeo and Tybalt:
"A plague on both your houses! They have made worms' meat of me. I have it, and soundly too. Your houses!" (Act III, Scene I)
The irony lies in the fact that Mercutio's curse, far from being an idle outburst, holds immense significance within the play's overarching theme. The feud between the Montagues and Capulets, symbolized by their "houses," has already been established as a long-standing and destructive force in Verona. Mercutio's curse seems to exacerbate this rift, fueling the animosity between the two families. His untimely death becomes a catalyst for Romeo's subsequent actions, which ultimately lead to the tragic denouement.
Therefore, Mercutio's curse underscores the play's central theme by ironically setting in motion the fateful chain of events that ultimately doom Romeo and Juliet.