Mercutio fights Tybalt because Romeo refuses to fight him after Tybalt insults Romeo. Tybalt, angered by Romeo's refusal and Romeo's secret marriage to Juliet (Tybalt's cousin), provokes a confrontation. When Romeo tries to keep the peace and stop the fight, Mercutio, frustrated by Romeo's "dishonorable" calmness and refusal to defend himself, steps in to fight Tybalt. During the duel, Romeo intervenes to stop them, but Tybalt fatally stabs Mercutio under Romeo's arm. This act spurs Romeo to avenge Mercutio's death by killing Tybalt. Mercutio's fight with Tybalt stems from his loyalty to Romeo and his unwillingness to allow Romeo to be insulted and dishonored without retaliation, despite Romeo's attempt at peace.
